The council issued eviction notices on the tenement-style building in June 1997. Some squatters accepted council accommodation, but others declined and are now involved in a legal wrangle with the building's owners, the London Borough of Lambeth. The remaining squatters claim that because they have lived there for more than 12 years, they have acquired tenants' rights.
Oval Mansions (eight blocks with 47 rooms), lies in the shadow of the gasworks by the Oval cricket ground in south London; the fourth floor has views over the cricket ground. It remains home to 100 people.
It was closed in 1979, not without good reason: there are wooden stairs, no fire escapes and none of the flats originally had showers or baths. There is a Dickensian gloom to the stairways and it is easy to stumble into pockets of subsidence. Faded gargoyles stare down from the tattered facade and the odd mattress pokes out of a window.
But the mansions are ripe for redevelopment. The prime location, next to the cricket ground and just 15 minutes' walk from the West End, has not gone unnoticed by Lambeth Council. It commands a potential sale price of up to pounds 2m to a developer with an eye for loft conversions.
"The council is perfectly within its rights to sell it but we want them to sell it to a housing association or to us," said Jane Jarvis, who has lived at the mansions for almost 10 years. "The community has been here such a long time it would be cruel to split it up."
Ms Jarvis, a part-time teacher, moved to the mansions in 1989. "I found it through a friend of my boyfriend at the time. It's all through word of mouth. I was pregnant and my boyfriend was a student. I didn't have anywhere to live and there was no likelihood of getting a council flat immediately."
The conventional view of squatters as "scroungers" could not be further from the truth, according to Ms Jarvis. "The public has the idea that squatters are the dregs of humanity. But squatters basically live in a place where no one else wants to live. We don't enter the flats with ladders or by candlelight. It isn't an easy option. People imagine you have a different lifestyle from them, but you don't," she said.
Built in the 1890s, the original occupants were either gas workers or nurses. It was almost unoccupied when the Greater London Council sold it to Lambeth in 1979.
After its closure, news of the mansions spread. It was particularly popular with immigrants from New Zealand and Australia. During the 1980s, as squatters came and went, the mansions were ignored by the council, seen for most of the decade as the original "loony left" authority.
But those days are over. Following a devastating report in 1995, which described the council as "an appalling financial and administrative mess", Heather Rabatts, the new chief executive, was given a free hand to balance the books. This has meant the council will no longer turn a blind eye to the Oval Mansions squatters.
Inside, many of the flats belie their external appearance. They are often spacious, with huge original fireplaces. Residents, who include artists and designers, have electricity and gas. At the back, a garden once tended by residents is now overgrown. "People have let things go a bit since the eviction notices came through. They felt they were pouring money down the drain," said Ms Jarvis.
John Burgess has lived at the flats for more than 12 years. "I didn't expect to be here that long. It has developed as a community," said Mr Burgess, who is unemployed. "People have put a lot of effort into giving the place a bit of colour in what was a bit of a dead part of London. Now I like it and I want to stay here."
Mr Burgess helped establish City Racing at the bottom of his flat, an art gallery built on a former betting shop. "If we were allowed to, we could develop this into a unique urban scene," he said.
Lambeth council says it wants the dispute to be resolved as amicably and as quickly as possible. "We have offered the squatters tenancies with a view to ending this dispute. Bearing in mind Lambeth council's extremely long waiting lists, we feel that this is a more than generous offer," said Donatus Anyamnwu, Lambeth's vice-chairman of housing.