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.