The most expensive flat in the world is on the verge of being sold - for £140m. The property, part of a development designed and managed by brothers Christian and Nick Candy, is in Knightsbridge - one of the smartest parts of London. At a cost of more than £6,000 per square foot, the two-floor apartment is within walking distance of Harrods and Harvey Nichols. It also has a tunnel connecting to the neighbouring Mandarin Oriental Hotel - where top chef Heston Blumenthal is soon to open a restaurant. The four-bedroom apartment has floor-to-ceiling windows, its own car park and access to all the leisure facilities that a discerning buyer would expect, like health spas and squash courts. For those who still have big money to spend, McLaren is opening a flagship showroom at the development with prices from anything up to £230,000 for a supercar. As part of the whole complex, copious security features are in place to protect those inside. They included SAS-trained security guards, bullet-proof windows, an air purifier to combat poison gas attacks and a "panic room", in which the owner can hide if under attack. Estate agents admitted the amount of money offered is a surprise They said, at one moment, there was an over-supply in the top end of the property market, then there was a shortage. Experts said many foreign buyers have moved into the British market because it represents good value, compared with other countries, due to the fall of the pound. Even so, £140m is enough to raise the eyebrows of even the mega-rich. The nationality of the buyer is not known but, inevitably, there is mention of a Russian oligarch or an Arab billionaire. While most of the block has been sold, 35% of properties are still waiting for new owners. The former office block site was bought in 2004 for £150m. The Candy brothers hit the headlines recently because of a legal action over plans to develop Chelsea Barracks. The Prince of Wales had pleaded with the company that owned the barracks to produce a design more in keeping with other buildings.