Flats in Chelsea are often compared with bars of gold - and in many ways the comparison fits. The stock of both is very limited and hard to expand. They're both physical assets, so you don't have to worry about counterparty risk. They both derive their value partly from intangible factors such as history and aesthetics, making pricing tricky. And they have both performed well during the financial crisis, thanks to 'safe haven' demand from a global elite.