In the 20s, the oyster-catching industry that made Whitstable the pride of the north Kent coast hit trouble. This was partly because of oyster blight, and partly because of the rise of the prawn cocktail. But the changes taking place today in Whitstable, and many other formerly neglected seaside towns, might be characterised as a rejection of the prawn cocktail and a reversion to the oyster. The reason is that the Down From Londoners, or DFLs as they are known, who are spending money and buying second homes in Whitstable, are pursuing a vision of salty authenticity. It's Treasure Island, Moonfleet, Swallows and Amazons, The Onedin Line and Patrick O'Brian novels, Daniel Peggotty and Little Em'ly living in the upturned boat at Great Yarmouth in David Copperfield. It's not the kiss-me-quick version of the seaside, but the shiver-me-timbers one, the middle-class one.

Turning up in Whitstable at brunchtime, I stepped into a cafe called Tea and Times. It had a rustic Mediterranean look, and its clientele tended to be broad-beamed, well-dressed people wearing sunglasses on their heads, drinking freshly ground coffee and confidently filleting the Sunday broadsheets. As I looked about me, I couldn't help thinking that this cafe should be in some fashionable part of north London - Crouch End, say - and after a while I became quite indignant that it wasn't in Crouch End. Why, it might have been picked up there and whirled to Whitstable on a tornado.

The first person I spoke to in Tea and Times was Ian Downie. He hastened to point out that he wasn't a DFL himself, he was from Faversham, but he liked the DFLs 'because they're interesting people and they put property prices up'. There again, Ian himself is 'in property', aiming to convert a building on the seafront into flats. 'You should have seen Whitstable before,' he added. 'It was very down.'

A little way along the high street is a delicatessen called Cornucopia. It was full of artichokes, anchovies, caramelised onions and other goods of the sort consumed by the ton in Crouch End every day. The young woman behind the counter said that there'd been two incursions into Whitstable: in the 70s the hippies had come; now it was the straightforwardly middle class. She suggested I come back and speak to the owner of the shop - he, too, was 'in property'.

I walked for about 20 seconds in the direction of the sea, and came to another delicatessen, even smarter than the first. A man in a white linen suit was saying: 'Two bits of focaccia, please, and can we have some of your black olives? We had dinner last night with some friends who live locally, and they said they were absolutely delicious.'

Not all the businesses in Whitstable reflect gentrification - there's still Rock Bottom Records, there's still Toddy's, where everything's a pound. It's about 50-50, and the divisions within the town are reflected on www.sunsettown, a website by and for Whitstable residents. It features a noticeboard hosted by the 'Moany Old Git', who generously offers to 'share his most hated things about Whitstable with you'. Mostly the moans concern those changing the character of 'good old Whitstable', in particular the DFLs, who tend to be addressed directly. The tone ranges from more-in-sorrow-than-anger ('Please come to Whitstable and enjoy all its charms, but go home at the end of the day,') to the frankly quite rude ('Why don't you pick on some other town that might actually like you?'). The DFLs, or those in the town sympathetic to them, often thought-provokingly respond: 'Everyone seems to have forgotten that half of Whitstable is descended in one way or another from people who came down from London.'

The value of Moany Old Git, is that he provides a chance to let off steam covertly. When I asked the one Whitstable DFL who'd speak to me - a fellow called David who works as a food writer and recently bought a house in the centre of town - whether he'd experienced direct hostility, he said, 'No'.

In Whitstable high street, the sun was now shining quite strongly, and the sunglasses had been lowered from the top of the DFL heads to their rightful place. Graham Greener was putting some flowers out in the modern-style flower shop he established a year ago, and that bears his name. The flowers in question were celosias, which I had never heard of. He said that if gentrification was happening in Whitstable, then it's happening all over the Kentish coast. 'Have you been to Broadstairs recently? Herne Bay? Georgian houses there are being snapped up like nobody's business.'

A couple of doors along, Viv Jones was getting down to work at the business that bears his name, a fish and chip shop. He was in some doubt about gentrification: 'We've been here for 40 years, and until five years ago we never sold a can of Perrier. Five years ago you could buy a little terrace here for £40,000. Now there's nothing under £100,000 - it's horrendous for my children.' (This, incidentally, is nothing. Kim Foster, of Foster's Property Services, says that a small converted boat house on the front is going to cost you about £315,000.)

My next stop was the beach, which has a melancholy beauty: dauntingly grey shingle and ghostly groynes, a dark blankness to the look of the sea. It's dominated by the Royal Native Oyster Stores, which is where the oysters used to be landed, and resembles a cross between an old railway station and a church. Inside it is all bare brick offset by racks of Champagne - more Islington than Crouch End, perhaps. Around the walls are pictures of men doing things to oysters a hundred years ago, and the clothes the men wore echoed the clothes I'd seen that day worn by the prosperous-looking people about the town: Breton caps, sun-faded smock-type garments - the difference being that the smocks of the DFLs come ready-faded. There is a cinema attached to the restaurant, and it was showing About a Boy, viewing of which is compulsory whether you live in Crouch End or Islington.

The restaurant-cinema, and the pristine Art Deco Hotel Continental are owned by the Green family. Until they got their hands on the Continental five years ago, it was so ungentrified, according to one local, that 'there wasn't just a fight every Saturday night, there was one every day of the week '.

I moved along to the harbour, which just now is finely poised. You can walk along the south quay inspecting the pretty seafood stalls, or saunter towards the Crab and Winkle restaurant, which is shortly to expand and develop a few holiday flats, but your Timberland loafers might just trip over a coil of rope as used by actual sailors, and send you flying into the actual sea. Wiz Cunningham, who was sitting in a harbourside office promoting the annual oyster festival, said a remorseless paradox was operating in Whitstable. 'People come to Whitstable because it's authentic: there's no promenade, no children's arcade - but the more visitors and second-home owners we have, the less authentic we become.' She thinks there's a danger of 'killing the goose that lays the golden egg'.

But I don't know about that. They killed the goose in Southwold in Suffolk decades ago, in the sense that the fishing fleet has long been down to about six boats. Nobody comes to Southwold because it's authentic, they just come because it's beautiful in that Peggotty way. Whitstable is the nearest nice coastal place you hit south of London; Southwold - Hampstead on Sea as it's known - is the northern equivalent, and the leading exemplar of the DFL phenomenon.

I myself often visit Southwold from north London, and I monitor my young son's attempts to describe the perfection of the place. 'It's like a Thomas the Tank Engine town' is his best shot so far, although in fact the narrow-gauge railway that connected Southwold to the imperfect world at large closed in 1929, helping to insulate it from seaside commercialism. A conflagration of 1659 was also useful, causing greens to be created as firebreaks, so that what you have today is a village-like town of elegant, mainly Georgian buildings, with a smart white lighthouse which dates from 1890 but looks as though it was built last week by Sir Terence Conran. There are about 12 amusement machines in the whole town and they're all on the newly restored and beautiful pier. There's no graffiti in Southwold and never any untoward noise, unless you count the jangling of the Morris dancers who often appear outside, and especially inside, the civilised and historically minded old pubs run by the town's brewery, Adnams.

My favourite is the Admiral Lord Nelson, which is full of old documents proving that there's been a pub in this site for 300 years. It's full of plummy men, saying contented things to each other like, 'Ah, you're just in the nick of time!' and 'Now that is fortuitous.' I saw one of these sorts of men being shown a loaf of bread by a woman in a headscarf. 'Wholemeal or seedy?' he querulously boomed.

It was in this pub that I first heard a conversation of the sort very characteristic of the town. A man with a Suffolk accent was talking to a man with a patrician accent. The former was telling the latter how he'd been trying to keep crows off a television aerial. The posh man seemed more vexed about the matter than the local, and I worked out that the aeriel and crows in question were on top of his Southwold holiday cottage, and that the local was caretaker of the property.

In the old days, according to Wiggy Goldsmith, town councillor, many locals would have signs in their windows saying 'apartments to let', and that's how it was done. But over recent decades, the DFLs have been buying up the town. Forty-five per cent of houses are now owned as second homes, and the full-time local population has fallen from a post- War high of 3,000 to about 1,200. This is what one Southwold shopkeeper is getting at with this sign on his door: 'This is the only mortuary in the country with a bus service running through it.'

Goldsmith, who is the grandson of a longshore fisherman, adds that: 'If you want to meet the people who cut hay around here, you've got to go to the other side of the A12,' which is his way of saying if you want to meet people who do real jobs as opposed to catering for tourists. They're driven there by property prices. Sonia Moore, of the HA Adnams estate agency, told me that 'a four-bed period house will cost £365,000. Five years ago that would have been £180,000.' Whitstable practically exploded when a beach hut there sold for £6,000. In Southwold, the choicest ones go for towards £60,000. Bearing in mind that these are 14ft by 14ft, don't have toilets or drinking water, and that you're not allowed to sleep in them, I suggested that this was rather too much. But Sonia explained that the typical buyer, far from being some sort of financial surrealist, would probably be the owner of a holiday cottage that he let out, and that a beach hut would increase the prestige and therefore the rent he was able to charge.

The locals are driven out of the housing market, but they can earn good money from the incomers, and both sides can agree that the town is beautiful, and they don't want to change the way it looks. As the sun goes down, and the candles are lit in the George and the Swan hotels, and the waiters stand tensely, their napkins draped over their arms, an uneasy truce seems in operation.

Apart from the fact that it is a small seaside town with a diminished fishing fleet, Whitby would seem to have little in common with Southwold. There are many things anchoring it in the ungentrified world: Sailor Sid's Tattoo Parlour ('Tattooing by Sid and Shane'), the exciting Saturdayish smell of fish and chips that always hangs over the town, and the fact that one helping of those fish and chips will cost about £3 as against £5 in Southwold. But the groundwork for a transformation has been laid, albeit accidentally, and over many years. When the coastal railway connecting Whitby and Scarborough was closed in the late-60s, a couple of railway cottages outside Whitby came up for sale, and my dad was urged to buy one. 'Come on, Bill,' my dad's mate said. 'I'll have one and you have the other - they're only 300 quid apiece.'

Whitby was an increasingly inaccessible, quiet town, in other words, and there was little commercial resistance to planning restrictions that helped preserve the beauty of the higgledy-piggledy little houses and shops rising up from the harbour.

Things are beginning to sound a little ominous now, aren't they? But in fact Whitby was still a quiet spot in the early 80s when I myself still lived in Yorkshire. There was no hippy invasion, like the one preceding the DFLs in Whitstable. Instead, the surprisingly gentle goths came (in Bram Stoker's novel, Dracula lands at Whitby in the form of a dog) and lovelorn Yorkshire sixth-formers like myself would journey across the Moors by bus and sit on the cliff next to the ruined Abbey, wondering whether to dash ourselves on the rocks below or go for a pint.

Then they made bloody Heartbeat. This series, which started in 1992, often features scenes set in Whitby and today, according to the receptionist in the Royal Hotel, 'Most people who come to Whitby come because of Heartbeat. ' Others, slightly more high-minded, might have read the Whitby scenes in AS Byatt's novel, Possession (which has just been filmed in the town), or those in any number of other recent novels. Even I've written a novel with Whitby scenes, for heaven's sake. The place is culturally fashionable and tourism is booming. It was given a further boost five years ago by another ersatz phenomenon, the visit to the town of the replica Captain Cook's first ship, HMS Bark Endeavour. This year, the Endeavour is back in the harbour, and Whitby has never had it so good. Images of the Endeavour are everywhere; there's even a small motorised replica of it tootling in and out of the harbour loaded with tourists, and a sound system booming out taped sea shanties. Granted, this would cause heart attacks among the occupants in the £60,000 beach huts of Southwold, but Whitby's success is also being accompanied by subtle gentrification.

Alison Robson, a co-owner of the Magpie Cafe, says: 'It used to be older people coming here, and they would have fish and chips, tea, bread and butter and a cup of tea. Now we get a different sort of customer and we find we have to serve wine.' Not to mention 'noisettes of lamb pan-fried and served with a port and redcurrant jus'. These developments are all recent. 'We weren't licensed until a couple of years ago,' says Alison. At the White Horse and Griffin hotel and restaurant on Church Street, a log fire burns nightly for the benefit of a beaming, middle-class clientele drinking very cold champagne at £5 a glass. The place gets more successful every year. 'Whitby is now the busiest town in the whole country,' June Perkins, wife of the proprietor, startlingly avers. 'And it's year round. Take a weekend in January - we'll be fully booked. It's a slightly different sort who come off- season,' she adds. 'Wealthier?' I ask. 'Yeah,' she says. When I was growing up in York, a wet weekend in Whitby was very much a minority taste. Now it's a standard option for connoisseurs of romantic melancholy.

The headland above the town is known officially as 'Whitby Headland', a package including the 199 steps, St Mary's church, the ruined Abbey, and the adjacent ruined mansion, now turned into a stultifyingly tasteful visitor centre by English Heritage. The interior, with its bare wood and discreet lighting, looks like a Hampstead living room.

Of course, house prices have gone mad. According to Laura Noble of Reeds Rians, a two-bed cottage with a sea view costs £135,000. Two years ago, that same house would have been £40,000. 'They're being used as second homes or they're being bought as investment. After all, stocks and shares aren't doing so well right now, are they?' The new prices, she admits, are beyond the reach of the locals. 'You've got to remember that the average salary here is around £10,000.'

I walked down a harbourside alleyway towards The Whitby Fisherman's Society and Football Club. As soon as I stepped through the door, one of the men at the bar spotted a leaflet in my pocket advertising holiday cottages to let. 'If that bloke,' he said, meaning the man who owned the properties on the list, 'didn't own so many houses there might be places for our children to live.' The tone was mildly peeved rather than angry, and in fact the businessman in question is a member of the club. 'He's fully paid up so there's nowt to be done,' said my informant. He then added that the people buying second homes in the town as opposed to renting were not generally DFLs as in Whitstable and Southwold. They were up from South Yorkshire - UFSYs in fact, which unfortunately doesn't have the same ring.

There's nothing stylish about buying a second home in Scarborough. Doing so will only confirm in your jealous friends' minds that idea that you're rather crass really. But Whitby, with its more oblique charms, its literary connections, is a different proposition. Accordingly, house prices in Whitby have recently outstripped those in Scarborough for the first time.

Scarborough is also being left behind by two other ex-fishing villages in North Yorkshire: Staithes and Robin Hood's Bay. In East Anglia, it's not just Southwold that the DFLs are targeting. It's Aldeburgh, Walberswick, even Holt and Cromer to the north. In Kent, besides Whitstable, it's Herne Bay, Ramsgate and possibly long-somnolent Broadstairs. And don't get me on to Padstow in Cornwall - Rick Steinville as it ought to be called.

Clearly this new taste for a subtler, slower kind of seaside pleasure is only a function of the primary cause: the high value of big-city property. This emboldens people to buy second homes in pretty seaside towns, or possibly promotes 'downsizing', causing city dwellers to move out and buy first homes in these same places.

When I asked him what could be done about it, I do believe that John Miller, mayor of Southwold, would have thrown up his hands in despair, had he not been holding two heavy shopping bags: 'What can you do? It's a fact, it's happened.' The government keeps hinting that it might slow down the property market, and is considering giving councils the power to charge owners of second homes full council tax, as opposed to half. (They've been able to do this in Wales for the past 10 years.) Meanwhile, the Moany Old Git, positive and go-ahead as ever on his Whitstable website, suggests: 'How about moaning about it as a solution?' It does seem to be the standard option.