Why do you think this rent is extraordinary? Unless you meant that it is extraordinary as a result of extraordinary purchase prices. As a proportion of purchase prices, these rentals are nothing special. Rental prices will lie within a relatively narrow band near the cost of maintaining a mortgage for the simple reason that if they get much higher, people can finance more property purchases and hope to gain on price growth, and if they get much lower, a substantial proportion of investors lose the ability to leverage and so the returns won't be attractive.

> If you walk through Knightsbridge in the evening, at a time when people in well-adjusted cities might be having dinner with their families and friends, or at least watching TV with their kids, you will see a wall of black, punctuated by the odd square of illumination.

Try walking around the wealthiest area of any city, and the patterns will be very different from "normal" areas: Flats will be larger, and someone at home does not necessarily mean the building will look very lit; more people have second homes, or travel a lot, or work late, and so an inhabited flat may still have people at home more rarely. You can easily find rich areas that look almost black even when most houses are occupied.

I don't know if Knightsbridge is an aberration, but I do know London in general does not in fact have a lot of empty properties in proportion to its size/population. I don't have a source handy, but the last time I looked at the stats it was small. More importantly, the vast majority was empty for other reasons than uninterested absentee landlords.