Which is why Skyhouse is causing a lot of interest, not least in Government circles. "It's being used a lot in sustainability and government thinking, not just here but overseas. It's been to New York and China," says Marks. In this country Skyhouse - along with other, more conventional new-generation high-rise housing schemes coming along now - might just offer worried ministers a way out of a classic political dilemma. On the one hand, lots of voters are worried about volatile house prices and want to live in or near the countryside. Allowing more - many more - homes there would appease that interest group. On the other hand, nimby voters already in the countryside - and urban dwellers with a romantic notion of the countryside who like to escape there - don't want to lose any more of it, and resent all the new estates swallowing up the fields. With reason. Look at the forecasts of how many hundreds of thousands of new homes are officially needed in crowded Britain over the next 15 years and - on the conventional suburban model - you are looking at the permanent destruction of vast swathes of the British countryside in order to accommodate our ever-multiplying, ever-smaller households. More and more houses with fewer and fewer people in them. This isn't really about population growth, it's about social trends and consumer choice.

Skyhouse offers an alternative to this and, as a design, it is particularly clever. Rather than imagine a conventional single fat tower, Marks and Barfield see it as a cluster of three slender ones, oval in plan, looking outwards from a central support column. It's as if a family of three people of different heights - father, mother, child - were all standing round the same grab-rail on a bus. By not having the three sections rising to the same height, that lets you do interesting things on the tops of the lower ones - like gardens in the sky. They borrow a trick from the latest generation of office towers, too. Commercial skyscrapers nearly always now have "sky lobbies" - lofty spaces arranged at intervals up the towers where you switch lifts if you're not going right to the top. In a residential building, these sky lobbies become the equivalent of village greens or town squares. And like any community - any village or group of conventional streets - the idea is to ensure a social mix.

There will be affordable housing for "key workers" - nurses, teachers - as well as well-off professional hankering after the penthouse-and-health-club lifestyle. Marks and Barfield have crunched the numbers on this and reckon it's entirely viable. A tower costing £164m to build, with 35 per cent key-worker apartments, will still yield a 20 per cent profit to the developer. And why not mix people up? In most Victorian streets, identical houses can contain people of all income groups, cheek by jowl. The whole problem with tower blocks in the past was that were not mixed socially, but confined to one class or another. When you look at the proposed interiors of the flats, the look of the low-budget ones is more appealing - to me, at any rate - than the vast ultra-luxury ones which start to feel a bit corporate, a bit office-like. Still, plenty of people will want to pay for them, and those that do will help subsidise the cheap ones.

Marks and Barfield are now exploring different versions of the design. The original idea was to go very tall - 60 to 80 storeys, say, up to 1,000 apartments. A slightly trimmed-down version followed, at 50 storeys, containing 400-600 apartments. "It's the same as a neighbourhood of around 800 people, the equivalent of about three streets," says Barfield. And the shape and height of Skyhouse lends itself to some low-energy-thinking: the design, which channels wind through its open centre, can incorporate helical wind turbines while all that glass can include a lot of photo-voltaic cells to generate more power. They've been in talks with a number of cities but London looks the likeliest candidate for the first Skyhouse. Probably a scaled-down version at first, a 214-apartment model that can fit onto a tight urban site of around 50 square metres. Deals are close to being signed. But you just know that they do not want to build too many miniaturized Skyhouses. They want to build the full-scale thing. In which case, they have to tackle some residual prejudice about what happens to people when they live up high.