For more than 30 years Martin Langdon and his brother Nigel have been building a city. Year after year they have worked tirelessly, shaping its skyscrapers and its public buildings, its apartment blocks, its cathedral and its boulevards. Between them they have mapped out its monuments and its plazas. In all that time the gloss of newness has begun to fade, and the pioneering utopianism of the early days has given way to a grittier understanding that what cities really need to flourish is a messy vitality.

Their city is established enough now to have suffered the not-so-tender mercies of urban renewal - expressways have been carved through some of its oldest areas. Some buildings have been replaced, or remodelled. And all of them, lovingly carved from Plasticine, sit on a board 15ft long by 4ft wide, marooned in a remote vicarage in North Yorkshire, where the brothers' clergyman father moved his family in the 1960s.

The model city has a bit of a problem with dust, and is hidden from threatening sunlight in a permanently darkened, locked room. In places its fabric is looking a little the worse for wear. More worryingly, the Langdons must find a new home for it if it is not to face destruction. The house is on the market, and their city has become a fragile cuckoo in the nest.

This was a project that began as a childhood enthusiasm. It grew into a diversion for two bright but isolated schoolboys who had to make their own entertainment in the absence of near neighbours. By the time they left home to become students, the city had taken on a life of its own, and there could be no question of simply sweeping it all away. They would work on it in bursts when they came back for holidays. "It became much more complex as time went by, and the board began to dictate to us," remembers Martin (above). "When we were children, we could just bung anything on. It became much more difficult as the city began to develop its own history. There are times when we have almost wished we could throw everything in the bin, and start over." Through the brothers' 30s and 40s they kept coming back to the city, which grew increasingly autobiographical, reflecting their travels in its inspirations and architectural expressions. This is no longer a glorified train- set. It has a powerful presence, the result of the thousands of hours that have been poured into its creation.

Martin, 45, trained as an architect, then became an art- ist. His 40-year-old brother is more of a gifted amateur at town-planning: he makes his living from designing greeting cards. Originally each had his own city, built in separate rooms in the house. The day came when they decided on a merger, but rather than constantly fighting, they have divided the city in two. "We have an end each; we're both total dictators in our own zones," says Martin.

In some areas the city is still a hymn to drip-dry 1960s modernity. "You have to remember what it was like visiting London in the early 1960s - there was simply nothing so exciting as seeing the cranes that were building Knightsbridge Barracks towering over Hyde Park." Other sections have a more traditional flavour. "Going to Rome was a revelation. So was Paris. I became a closet Baron Haussmann, looking for ways of driving great boulevards through the existing fabric. Later it was New York that began to infiltrate."

The result is a model that reflects the way actual cities grow. There is a historic core, with organic street patterns, and a 20th-century section with a grid layout. And it has been long enough in its gestation to have apparently grown old and changed its mind, rather than present a single fleeting snap-shot of urbanistic adolescence. "That is dangerous of course: you always run the risk of going too far, and destroying the city with the new developments."

What really drives Martin is not so much the chance to make miniature architecture, but the spatial character of cities. "What I really hate are suburban cul-de-sacs, and dead, pedestrianised towns." For anybody who shares that vision, the Langdons' model is, despite its origins, the paradigm of a surprisingly grown-up view of the city.