the knightsbridge flat has been put on the market for a fraction of the price of neighbouring homes , at £ 575,000 .
it has only three years left on lease putting cost at £ 15,000 a month - going rate for renting a three-bed in the area .
property experts said if a buyer decides to extend the lease , the property could be worth upwards of £ 6 million .
but renewal would not come cheap with the cost for a new 90-year lease estimated at between £ 3.35 and £ 4.5 million .