It is one of London's most exclusive addresses. Michelin-starred restaurants are just a block away, the American embassy is around the corner and Hyde Park is at the end of the road. To share the same postcode ought to cost millions.
But the new residents of 18 Upper Grosvenor Street, a raggle-taggle of teenagers and artists called the Da! collective, haven't paid a penny for their £6.25m, six-storey townhouse in Mayfair.
The black anarchist flag flapping from the first-floor balcony gives a clue what they are up to: ever since finding a window open on the first floor on October 10, the group have been squatting in the once opulent property, and only plan to leave when they are evicted. This might take some time given that after almost a month, whoever holds the deeds - a company called Deltaland Resources Ltd, according to the Land Registry - doesn't appear to have noticed that their multimillion-pound building has been taken over.
Behind the white pillars and imposing wooden door of the grade II-listed residence, the 30-plus rooms are now scattered with sleeping bags, grubby mattresses, rucksacks spilling over with clothes and endless half-finished art installations. While their neighbours' walls are lined with priceless paintings, No 18 now exhibits a room full of tree branches and another with a pink baby bath above which dangle test tubes filled with capers. Spooky foetuses line one fireplace.
The group are seasoned squatters. Over the past few years, they have enjoyed some impressive central London addresses - including two on Kensington High Street. But their latest home is "by far the most grandiose", said Stephanie Smith, 21, one of the group, under a chandelier in the downstairs drawing room.
They had been watching the building for "at least six months" before they decided to try moving in, she said. "We had put tape on the keyhole, and kept looking through the letterbox to see if anyone had been there." Then, one October night, five of the group decided to go in. Some of them wore high-visibility jackets to look like builders; Smith had a clipboard and fur coat. They propped their rented ladder up against the front of the building, and one man climbed on to the dilapidated balcony.
"I went across to the window and I couldn't believe it when it was unlocked," said the squatter, who declined to give his name. "I was so happy. We didn't really expect it to be open, so it was a really exciting moment."
Almost a month since the occupation began, no one from Deltaland Resources Ltd, which is registered in the British Virgin Islands, has been in touch with the artists; if they call around they will find the locks have been changed. The Da! group have reconnected the utilities and say they will be paying bills.
Smith insists they have done nothing wrong. "Squatting is not a criminal offence, it's a civil matter," she said. "If the owners want to kick us out they will have to apply for an eviction notice at the county court.
"If anything, we are improving the building by mending leaks and things like that. The building is listed so English Heritage might be interested to see how the owners have let it disintegrate."
The group has had a mixed reception from the other residents of Upper Grosvenor Street. "Our next-door neighbours have been really nice; they've even let us use their wireless internet," said Smith. Another neighbour, a man called Alexander, has offered the services of his maid to cook them food, she added.
But not everyone is happy. Especially not the proprietors of a new restaurant opposite, Corrigan's, run by the Michelin-starred chef Richard Corrigan, which was due to open last night. Jacques Dejardin, the restaurant manager, was horrified to discover earlier this week that his upmarket location was directly opposite a squat.
"It's rather bewildering. When you move into an address like this you don't expect to have squatters as neighbours," he said. He needn't worry about the squatters popping in for dinner: they are all firm devotees of freecycling, and collect all their food from supermarket skips.
But Dejardin might not be too pleased to learn that tonight the squat is hosting a party. From 7pm to 11pm, the Da! gang will be projecting images on to each of the 19 windows at the front of the squat. "It's going to look like a doll's house," said Smith, "and there is going to be a harpist and a cellist and performance artists."
The squatters have no intention of going anywhere, and because of squatters' rights, no one can move them on until the owner takes them to court.
The group will have to stay a long time before the building becomes theirs - squatters can only claim ownership of a dwelling after 12 years if the original owner hasn't tried to get it back.