Leaks aren't always visible - remember the one that apparently washed away the South Circular and shut London Road for a while. And Thames Water lose litres a day so I'd say it's highly likely that there are a few leaks around Forest Hill.

In fact, there's another slow leak on the South Circular at the moment outside the old barracks which I've reported twice to TW over the last 3 months. The water wells up from under some new tarmac that was put down to fill a large pothole in the bus lane. But since I can't give them the exact postcode, it appears the telephone bods can't pass it on properly.