“Parents and children revel in the sense of freedom, how they can run wild in such a beautiful landscape. There’s something very knowable about an island like Tresco; its pleasures are simple, but they are precious.”

There is something uniquely beguiling about the Isles of Scilly in general – a scattering of around 200 separate islands and rocks, five of which are inhabited – and Tresco in particular, once famed for the flower industry.

Part of tbe Duchy of Cornwall, the main island St Mary’s is reachable by helicopter or Twin Otter Plane and, in season, by boat, it’s remoteness lending a thrill and an exclusivity, not least because in high season travel alone can cost upwards of £700 for a family of four. Then a water taxi takes you to the hermetically sealed world of Tresco, which is just two and a half miles long and one mile wide yet boasts a landscape ranging from gusty heathland in the north to the subtropical Abbey Garden in the south.

The maritime microclimate (locals roll their eyes if you mention the Gulf Steam, which is miles away) means ginger lillies are in flower, fat pink camellias in blossom, extravagant South African proteas thrive alongside spikey South American bromeliads and sculptural aeoniums. Not to mention the profusion of narcissisi and daffs strewn over every hillside and grass verge.

The island has 150 residents, the majority of whom are employed by the family-run Dorrien Smith estate, which has the leasehold. Charles and Diana took their boys here. Harold Wilson was once a Scilly regular. These days it’s Jude Law and American Anglophile Bill Bryson, who often sings the praises of Tresco, which comes as no surprise given he’s a sucker for a gentler, bucket-and-spade, homespun Britain that scarcely exists any more.

Except on Tresco. Tresco is Boden and Blyton and every idyllic childhood summer you ever had suspended in time; free of cars and crime, stranger danger and street lighting, where doors remain unlocked and the Proustian tang of a fresh crab sandwich will imprint on your palate and haunt your culinary dreams forever.

Occasionally there’s a Truman Show sense of staginess to its pinch-me-I’m-dreaming perfection: the sight of ripening lemons on a tree. In early March. In Britain. The friendly natives, the plump pheasants dimly wandering down the thoroughfares, a jet black cat draped languorously over a low wall, before disengaging himself and, right on cue, crossing your path. The white sand glittering in the sun with tiny grains of mica and feldspar, and the sea, I swear, genuinely turquoise. In early March. In Britain.

“A certain type of person comes to Tresco, because it’s not cheap,” says Anna Parkes, who runs the Tresco Gallery, where an intricate silver pendant sells for £243. “They are sophisticated, they’ve travelled the world but they come back because they want to buy into the community spirit, shared values, and if there’s a fête or a bingo night, they’ll be there. You see visitors stepping off the helipad and literally shaking off the mainland stress.

“We’re an old-fashioned island and we’re proud that people feel they’ve stepped back in time. Things are modernised and there’s wifi and so on, but the fabric of this place has stayed the same.”

Parkes’ husband, Steve, is the island’s estate gamekeeper and woodsman and one of several locals who have cameo roles in Archipelago, in his case accompanied by his springer spaniel, Buster.

Much of the movie was improvised and is so naturalistic as to make squirming audiences feel like eavesdroppers on a family struggling, in vain, to recapture the remembered, but elusive, magic of childhood.

The couple attended the film premiere in London. “The thing that struck me was the absence of any soundtrack; instead, natural sounds are amplified. It was Tresco in the raw,” she says.

The upper middle classes are easy to parody on the big screen, variously portrayed as brittle Gosford Park aristos or Four Weddings and a Funeral Hooray Henrys. Archipelago is very different; more meditative, and to be honest, entirely at odds with the messing about in boats, snorkelling with seals and hearty yomps for which this neck of the water is famous.

Alison and Pete Malcolm, both 48, who divide their time between the family home in Oakhampton and San Fransisco, where he runs a software company, visit Tresco three times a year and cheerfully declare themselves to be “addicted” to its quietly powerful charms. She once took a helicopter to the island for a haircut. He comes in winter for the shooting. Their four children, Ellie aged 23, Izzy, 20, Phoebe, 14, and Giles, 12, adore the place.

“The whole Boden-on-Sea label might be true in the summer but off-season we die-hard regulars wear Sea Salt, which is a Cornish clotheswear brand, or 49 Degrees, which is available on St Mary’s,” observes Alison with a knowing chuckle at the de facto uniform of the middle classes on manoeuvres.

“We’ve been coming to Tresco for nine years and we’ve just had the children over; the first thing they wanted to do was sit on the pier with crabbing lines, catching them, counting them, then throwing them back,” she says.

Across at the outdoor cafe in the Abbey Garden, head gardener for the past 16 years Andrew Lanson holidayed on Tresco aged 14 and vowed to return and work here. He too appeared in Archipelago. “I am very curious to see the film, my only concern is that it might not show the island in the best light because the weather can be grim in winter.”

But the cognescenti know how to make themselves at home however hostile the elements. Kate Moore, 31, runs Tresco stores, a crammed emporium selling everything from sewing needles to wild boar salami. Dressed in Whistles and Kurt Geiger heels, she is the most glamorous grocer you are likely to meet, this side of Myleene Klass in an M&S advert. “Our customers don’t come to Tresco for a bottle of Jacob’s Creek and a tube of Pringles,” she says, matter of factly. “They expect something different and local: Camel Valley Cornish wine, ice cream from the dairy herd on St Agnes.”

Needless to say this carefully preserved heritage doesn’t come cheap. The accommodation on Tresco comprises upper-crust time-share or self-catering cottages so impeccably furnished Johnnie Boden himself would be happy to kick off his espadrilles and chillax. At the Flying Boat Club, the breathtaking beachfront chalets are a tasteful mélange of Designers Guild curtains and Egyptian cotton sheets. In high season paradise costs an eye-watering £4,500 a week.

“The unique appeal of Tresco actually lies in what we haven’t got,” says Ian Warren, 43, deputy general manage of the hotel. “There’s an absence of noise and light pollution, nightclubs, litter and above all speed. Nobody goes anywhere fast and that's very rejuvenating. I lived here six months before I even found the key to my house, much less used it.”

But there are occasional voices of diquiet among those who feel uncomfortable about the feudal nature of a community where most people work for, and are housed by, a single landowner. Warren, who has a three-year-old son and whose wife works part-time for the estate, sees Tresco’s shared ethos as a strength, not a weakness. “There’s a collective sense of responsibility. If you see a discarded chocolate wrapper on the ground, you pick it up, no matter who you are.”

Discarded wrappers, on Tresco? About as unthinkable as a duff bottle of Taitinger. Sneer at the Dubarry boots and artisan pasta if you must, but here is a tranquil little corner of Britain that is forever Boden. And let’s be honest, who among us, hasn't dreamed of that?