Our destination is silted up, so we have to trespass on the ancient and rather grand pier of an Edwardian villa owned by a food magnate - I half expect to see Jay Gatsby standing at the end. As we tie up, glaucous jellyfish float downstream and packs of dog-like grey mullet circle round the pier's stanchions. It feels as though we've slipped back in time. Ahead, just visible through a screen of sentinel oaks and overgrown shrubbery, is a glimpse of what we've come this far to see: Spence Cottage. The name is ironic: the house is about as far from thatch and rose-trimmed trellis as you can get. Sitting slightly ominously on the soft verdant banks of the river and cloaked by trees, it is an uncompromising, monolithic piece of hard-edged Sixties modernism transplanted to the Hampshire countryside.
The house was built in 1960, when the newly knighted Sir Basil Spence was at the height of his fame. Spence, born in India of Scottish parents in 1907, trained as an architect in London and Edinburgh; he assisted Lutyens with drawings for the viceregal buildings at Delhi. After an heroic war (he was twice mentioned in dispatches), he emerged as the leading postwar architect, designing pavilions for the Festival of Britain in 1951 - the year in which he began work on Coventry Cathedral, his greatest achievement. Spence would go on to design buildings for Southampton and Sussex universities, a crematorium in Edinburgh, and the controversial Knightsbridge barracks, ridiculed at the time for providing first-floor accommodation for cavalry horses. By 1960, he was president of the Royal Institute of British Architects (RIBA), and his crusade for modern architecture had earned him the nickname of "Saint Basil". Spence died in 1976, and his reputation took a nose- dive directly afterwards. Only now, as mid-century design swings back into fashion, is his work being re-appraised.
Spence Cottage was inherited by his son-in-law. Two owners later, in 1986, Ari Zaphirou-Zafiri, a financier and architecture enthusiast, walked into a Lymington estate agent and described his ideal country house, and gave them a price. The agents replied that they had the house, but not the price. None the less, he viewed the place, and after two hours' walking around it, fell in love with it. When he bid for the property, he discovered that his competitor planned to pull the place down. Zaphirou-Zafiri's love for the place won over the vendor, secured his bid, and the house.
Despite its chain of ownership, the house has changed little since Spence lived there (although a pair of unsightly solar panels have been set up on the roof). The paint is peeling and the pool cracking up, but this faint decrepitude only adds to the building's mystique. It's as if Spence might still be living there, masterminding the remodelling of postwar Britain, an architectural Dr No.
The owner, who has an expanding family, has commissioned architect John Pardey - who has built his own house at Lymington - to update designs made by Spence for an extension to the house. Pardey is a vocal enthusiast for Spence's work. To him, the clean geometry of the house is nothing less than "poetic - the human face of modernism". "My generation vilified Spence," says Pardey, "and he's still the most unfashionable architect of his period." This is partly because, as a "gentleman architect", Spence "didn't play the social agenda". In this context, Spence Cottage starts to look like the indulgence of a visionary or an autocrat ensconced, god-like, in his rural retreat.
From the outside, the building is solid, slab-like, almost as though cast in concrete. Yet the upper storey of the house is almost entirely wood, a cedar-boarded block sitting on a brickwork base, cut into by angular blocks of chimney and outside stairway. You ascend stairs and enter at the first floor, through an authentically cadmium yellow front door, into an interior straight out of The Avengers. A broad horizontal slit of a window looks out on to the river, reflecting light off the softwood- boarded walls (this used to be an open balcony, but Spence had it glazed to make it a more useful living space). At one end of the 37ft room is a monumental, gravity-defying fireplace. Suspended over a curved base, the top half hangs in mid-air - half a ton of concrete supported solely by the brick stack above. It is an extraordinary installation, constructed by Spence's collaborator, the Danish engineer Ove Arup (famously responsible for another modern waterside statement - the Sydney Opera House); to achieve the characteristic pitted finish, a bush-hammer was brought down specially from the Coventry Cathedral site.
Set flush into the wall is a series of cabin-like doors which open into three bedrooms and tiny sauna-like bathrooms still with their original fittings. It is almost like being on board a luxury yacht (Spence was a keen boatman); with its cedar boarding on the walls, afzelia wood floor and redwood ceiling, the whole place feels nautical, a ship about to launch itself on to the river ahead. In Spence's time, the interior was filled with Danish wooden furniture: the only colours were occasional slabs of ultramarine on the brickwork, and a bright vermilion sofa.
At the far end of the living room, a spiral staircase connects to the ground-floor kitchen, utilities and patio under the cantilevered bay above, with a barbecue set into the wall like an enormous letterbox. The asymmetrical pool - now covered up - once featured a radiating eye motif designed by John Spence (a painted sketch for it was discovered by Pardey in the boathouse), its cornea a spinning car hubcap that appeared to blink in the water.
There's a flamboyance about the place that reflects its creator. A dandy in his goatee beard and bow-ties, Spence was a Sixties socialite. Lord Montagu and the yachtsman Uffa Fox partied here; guests were known to jump from the first-floor window into the pool. One dived off the top of the roof, and nearly brained himself on the winking eye. After that it was removed.
The house may look simple, but the details are lavish: elliptical door knobs, a steel suspended shelving unit and, on the studio Spence had built for himself in the landscaped grounds, his trademark aluminium guttering, angled and textured, like something out of a Sutherland painting.
The studio itself began life as a kit-bought summerhouse which Spence customised, adding an angular extension. A retreat within a retreat, it sits at the rear of the house, in the three-acre site designed by Fifties landscape artist Sylvia Crowe. But her planting of bamboos, rhododendron and camellias is overgrown. A pond has completely vanished, as has the Geoffrey Clarke aluminium sculpture which stood at the end of the terrace - itself cracking up and threatening to slide down into the Beaulieu river.
The New Forest is not known for its contribution to modern architecture - its difficult to imagine Spence getting planning permission for such a house nowadays. Indeed, Pardey's plans to extend the house have met with opposition. Whatever the locals thought of Spence and his lifestyle, they may have been mollified by his gesture to village planning. At the top of the lane which provides access to his house, Spence designed a bus stop in Festival of Britain mode with a verandah-like awning.
Deluxe and delightful, a dream home without the heartache (unless you count the million or so which needs to be spent on its refurbishment), Spence Cottage is a Scandinavian house in the woods, or something out of a Hitchcock movie, a Bel Air villa. All it needs is Bryan Ferry in a white tuxedo, or Noel Coward's Live In Las Vegas album on the Dansette. But for all its American or European influences, it's also quintessentially English, tying up Forties neo-romanticism with postwar modernism, sitting in its landscape like Hampshire's version of Lloyd Wright's Falling Water house. Put this place in a Chicago suburb and it would be a national monument.
A new generation unblighted by Sixties brutalism is already radically reassessing such design: this is the kind of house any modern design fan would wear their Prada loafers to the ground to find. But they'd be too late; its already spoken for. John Pardey's ambitious scheme of renovation and extension is already under way, and Spence Cottage is about to carry Saint Basil's crusade into the new millennium
Spence Cottage is featured in `Travels With Pevsner: The Buildings of Hampshire', written and presented by Philip Hoare, showing on BBC2 tonight.