It wasn't the usual London journey. As we cut up our third white van driver in as many minutes, he smiled sweetly, just like the others. An hour later, a leather-clad motorcycle courier pulled up alongside us at traffic lights to ask: 'What kind of drugs are you people on?' He had a point: the latest tour of the capital really is quite strange.

It takes place on a motorised four-poster bed, in which I was tucked up under a purple duvet next to a female friend and a chauffeur. As people look for ever quirkier experiences to mark special occasions, this is the wackiest addition to a market worth £60 million a year.

After cocktails in Mayfair, we were strapped in next to our driver Edd China, sporting goggles and flowing night cap. With the steering wheel between his legs under the duvet, turning corners promised to be a vaguely obscene act.

We were about to become members of the yard-high club, and within seconds of heading off down Piccadilly, the public started to comment: 'Leave her leg alone.' 'Where's your nightie love?' People stopped and pointed; an open-mouthed businessman walked into a rubbish bin. If you've ever wondered what it feels like to face the paparazzi, this is your chance. Camera flashes followed us up to Soho and down to Westminster, with sightseers turning their backs on Big Ben to snap our bed with its VW chassis.

'I think it will appeal to the slightly extroverted,' said Edd, as yet another grinning cabby politely gave him priority. 'You're going to be in people's pictures, you're going to be famous for 15 minutes. It's a Warhol thing.'

Depending on your personality, the tour is either a wonderful ego trip or brutal exposure therapy for the terminally nervous. But that's far from its only impact. As we hit 40 mph down the Mall, I realised London had taken on a totally new face. Everyone but everyone grinned, traffic wardens stopped writing tickets, strangers shouted encouragement. Initially it was slightly unnerving, as if someone had put Prozac in the water supply, but by the time we hit the King's Road I had started to enjoy the Santa Claus effect.

The one exception to the love-in was a pompous policeman on the King's Road, who flagged us down, initiating a surreal conversation.

Edd: 'Can't stop long, it's a double yellow.'

Officer: 'Do not take the piss sir. Get out of your vehicle. My colleagues are questioning its safety and legality. What exactly is it?'

Edd: 'It's a VW Variant, officer.'

Officer: 'To my knowledge, Sir, VW do not manufacture a four-poster bed.'

He asked increasingly desperate questions, eventually picking on the height of the headlamps. Edd, with a degree in engineering design, was unfazed. He is, after all, constantly stopped by the police. 'My record is four times in one day, 19 in a week,' he said. 'I'm thinking of charging extra for the true London experience.'

The tour is part of a one-day package which also includes a flight over London in a tethered balloon and a trip on a motorised sofa. But today the bed was the star. I might have been under a duvet, but this was the best fun I've ever had with my clothes on.

• The Outrageous Tour of London costs £195 for two. Contact Red Letter Days on 0870 444 7000 www.redletterdays.co.uk.