Ancient oaks and luxuriant lawns stretch away from a spectacular Palladian mansion. A crescent-shaped lake glints amid trees in a shallow valley. People linger over picnics and wander off to woodland glades, kids kick balls around, soul music washes over us and people emerge from the mansion holding glasses of wine.

This isn’t a summer function at a stately home but a “new” municipal park in south-east London, nine miles from Trafalgar Square, on the cusp of its official opening on 20 July. The opening coincides with a week-long festival marking London becoming a National Park City (20-28 July), a Mayor of London initiative designed to make the capital greener and healthier.

So where has all this green space been hiding? Beckenham Place Park was a public golf course for more than 100 years; in the 1990s it was among the busiest 18-hole public courses in London, perhaps Europe. Sporting legends, such as footballer Ian Wright, cut their golfing teeth there, as did less-stellar sports enthusiasts, such as myself.

But in the late 2000s golf seemed to lose its allure and the local authority decided – amid much protest – to close the course and restore the park for the benefit of the wider community. As the course died its slow death, dog walkers would roam the fairways. The course finally closed in October 2016.

Like many, I assumed new flats and supermarkets would shoot up on the site but this was never Lewisham council’s plan. Now the 96-hectare space, one of London’s largest council-owned parks, has been remoulded as a recreational and natural attraction with the help of around £5m of Heritage Lottery grant. And already it’s attracting greater numbers than previously, drawn by the freedom to roam green spaces, and by the happy ambience at the mansion, with its groovy music, pop-up bar, yoga, arts and crafts, and vinyl store.

But the jewel in the park’s crown is the new 283-metre-long, 45-metre-wide lake, a recreation of an original Georgian water feature. Its source, says programme manager Gavin Plaskitt, is a combination of an aquifer reached by a new borehole, ground water and an existing stream. Reeds and other aquatic plants absorb unwanted nutrients in the water and oxygen pumps keep the water, which at its greatest depth is 3.5 metres, healthy. A beach area neatly shelves into shallow water for younger children to splash around in.

On recent balmy evenings some locals, tired of waiting for the official opening, pushed aside the barriers to take to the inviting water. One older man, Alf, told me: “I only moved here a couple of years back. I can’t believe my luck with a place like this opening up on my doorstep. I’m over the moon, but I want it all to myself!” A former triathlete, iron man and boxer, he says the new lake will encourage him to get back into regular training. “The water is superb,” he says, “but I hope they do a deal where you don’t have to pay much if you’re over 60.”

With its lake, a 5km figure-of-eight running route and cycle tracks, the park should become a centre for triathletes. It is holding its first triathlon in October. From next week, swimmers will pay £3 a session. There will be Canadian canoes for hire, and paddleboarding lessons and sessions are in the planning, too.

The initiative is accompanied by a major effort to improve and bolster natural habitats. East of the lake, a wooded wetland habitat is being created, a rewilding venture aimed at creating a fauna- and flora-rich environment, and boosting biodiversity. A raised path alongside it leads deeper into the woods. “Already we’ve seen dragonflies, and birds like little egrets turning up,” says Plaskitt, who is now watching his vision of six years ago bearing fruit.

Throughout its golf course years, the park was a well-known natural space, carefully tended by the Friends of Beckenham Place Park. Among its natural treasures are ancient trees (including wild service, turkey oak and wych elm), swathes of bluebells and birds such as firecrests, woodpeckers and owls. But now ponds are being cleaned out and wildflower meadows are taking over what used to be fairways. During my visit, the cackle of the ubiquitous local parakeets was the prevalent birdsong – as it was during my golf rounds, when they would appear to be loudly mocking my wayward shots into the rough.

Dave de Silva leads birdwatching trips in the woods and along the Ravensbourne river, which runs through the east of the park. “Part of me feels a sense of nostalgia. When it was less well-used, the place had a feeling of space and emptiness – unusual round here.”

He admits it’s taking a little time for him to get used to seeing people roaming all over the grassy areas and, as a birder, fears that music from the mansion might not be conducive to the autumn migration watch, but “the new habitats will definitely attract more species”.

De Silva is, however, a fan of the new gardens, still a work in progress: “They’re looking brilliant.”

I walk with Plaskitt up to the gardens, past an attractive new adventure playground. Beyond is an 18th-century stable block and homestead cottages with clock tower, now restored after a fire in 2011. Here, there are plans for another cafe.

Plaskitt looks on as volunteers busy themselves with mulch, seedlings and wheelbarrows. He remembers a time a few years back when there was “a crazy level of opposition”. Now, though, he feels the level of community participation shows that the ambitious plans have been accepted. “The park wasn’t being used that much. There were periods when there were only 45 golfers a day on average, and hours would go by when we’d only see three or four dog walkers here.”

There’s plenty more to be done though, and funds to be raised. “One of our next priorities is to renovate the Foxgrove Club building [a large mock-Tudor house that has fallen into disrepair],” says project manager Alison Taylor. Then, in the flatter part of the park, there’s the Ravensbourne river, home to kingfishers, herons and grey wagtails but unkempt and unlovely: “We want to make more of the river – reattach it to the landscape, rewild sections and create flood storage zones,” Taylor says.

There’ll also be a much larger playground here, a magnet for kids on this side of the park, which borders less well-off neighbourhoods such as Downham and Bellingham.

I stop for a beer back at the mansion before cycling home slightly in awe of the project’s ambition, remembering how, in the 1980s, many of London’s open-air swimming pools disappeared and playing fields were sold off to developers. But at Beckenham Place Park it feels as though these negative trends have suddenly gone into reverse.

I may still clear the cobwebs from my golf clubs from time to time, but I’ll be needing a decent wetsuit for future visits to the park.

• Book swimming here. More information on events at the mansion at beckenhamplace.org. Beckenham Place Park is on the Green Chain and London Ring walking routes. Three railway stations serve it: Beckenham Junction, Ravensbourne and Beckenham Hill

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