We arrived at 7pm to be told, politely but firmly, by a beautiful girl in a fur coat that opening hours ended at four. But, lady, you are an anarchist and this is a squat! Aren't temporal formalities for squares, bread-heads and other sadsacks - like me?

When a young friend mentioned he was visiting the “posh squat”, a Mayfair mansion being occupied by a collective of artists, I begged to go too. He wanted to view the mind-extending conceptual art created and exhibited in situ; I fancied a shuftie around a 30-room, £6million house in that most silent and impenetrable tract of rich London, between the American Embassy and the five-star hotels of Park Lane. Besides, owned by the Abu Dhabi Royal Family, it ought to have more blingy bathroom fittings than Uday Hussein's redoubt.

So when a willowy chap called Ben agreed, despite the hour, to give us a guided tour my brain added a property-porn counterpoint to his discourse on alternative living. Just look at the cornicing, the high ceilings, the staff quarters, that graceful balustrade...Madonna's joint (a few streets north) must have looked like this before she did it up.

You have to respect the style of posh squatters. They choose the best postcodes. And they don't burn your floorboards, smoke crack or defecate in stairwells. Neighbours concerned for the recently squatted Fife House in Brighton, a Grade I listed Regency treasure, complete with a lavatory installed for the visit of Edward VII, probably have no reason to fear. Certainly, the Mayfair crew had proper middle-class reverence for original features.

Indeed, they seemed to cherish this elegant building far more than its owners. Ben pointed out a brown ceiling stain, a long-neglected leak that the squatters had fixed. After entering, quite legally, a month earlier through an unlocked window dressed as workmen, they'd had utilities reconnected and established house rules. And as we went from room to room filled with art mostly made from stuff found in skips - a papier mâché whale's tail protruding from a fireplace, a labyrinth constructed from a pile of doors, a chamber festooned with tights like a serial killer's lair - Ben tutted crossly at those who had breached regulations by painting on the walls.

At a happening the previous night, two female artists, dressed only in swimwear, had spattered each other with fluorescent paint and writhed around on the floor. Their Jackson Pollocked bikinis were now on display. It must have been, er, quite a spectacle. “It was very childish,” said Ben primly. “This paint will never come off.”

Until now, even as a strapped student, squatting had not appealed to me. It is my working-class aspiration never to be dirty, uncomfortable or have my privacy invaded by random, flaky strangers. Squatting ranks with music festivals and camping as something I can't believe anyone would do by choice. But this squat was different. No smoking, no drugs and a disciplined work ethic. Every morning at 8am, Ben ran round the house banging on a frying pan to rouse any slackers.

In what appeared to be the squat's operations room, an upper-class chap sat reading a Penguin Classic. Someone else surfed a laptop - friendly neighbours had let him piggyback their wi-fi. From a distant room came the sound of a piano and several pretty young women wearing a great deal of eyeliner flitted about. Any moment Violet Trefusis might walk in and start frotting with Vita Sackville-West.

But the Bloomsbury brigade had servants. Who cleaned the bathrooms here? Well, Ben said, everyone tries to give the loo a bit of a brush, but actually there weren't any bathrooms. The house had latterly been used as offices, so they'd all been removed. So how...? “Well, we don't wash very often,” Ben said brightly. “After a while you really don't notice.”

It was in the basement kitchen that I finally realised I'd never make a bohemian. Every surface was covered with crumbs and unwashed plates. The bin overflowed on to the floor. Ben, mate, you're going to get rats. He agreed, but added that at least they all ate well. Every night an insomniac artist took a shopping trolley around Mayfair collecting discarded food from the bins of Pret a Manger and M&S. The fridge was crammed with luxury ready-meals a day past their sell-by date. They gave surplus sandwiches to armed cops guarding the US Embassy. For breakfast they collected a bin-bag of slightly stale pastries left outside Paul's patisserie near Harrods. You could live like a prince, Ben said, on all this waste.

But the greatest waste of all was the house itself, left vacant for several years by a letting agent, registered in the British Virgin Islands. The Empty Homes Agency, a charity that tries to persuade owners of Britain's one million vacant properties to let or sell them, believes that the Abu Dhabi royals have many equally grand empty properties across London.

Indeed the more magnificent the mansion, the more likely it is to sit empty: the fake Tudor palaces of The Bishops Avenue, the near-statelies of Kensington Palace Gardens, always look dark and deserted apart from the odd resident Filipino maid, their masters wintering in Gstaad or the Gulf. These houses are not homes, only assets, insurance against economic misfortune or political downfall.

Nonetheless, the owners wanted the squatters out. And I went along the following Tuesday to the eviction hearing to see the entire squat turn out in homespun Victorian garb: crinolines made of brocade curtains, cardboard wing-collars, battered top hats and junk shop tailcoats. A suited young man trundling a pull-along suitcase had come to watch: he'd just got a job with a TV company and figured joining the squat was his only way to live in London on £13,000.

The magistrate, a wry, kindly man, rejected their grounds for appeal, that it was “very cold outside”. And the band trailed back along Oxford Street in their bizarre garb like survivors of some period-costume apocalypse.

But Ben was undaunted. Eviction was just the final act in their living protest. They had another vacant house staked out, grander even than this one. Soon the Mayfair mansion would be made squatter-proof with iron shutters and deadlocks, still unoccupied but, it pleased Ben to think, with their art now sealed inside.

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