I noticed that you see the tracks start on the small ledge above the snow, moves from one corner to the other before hopping back onto the ledge in the same area and leaving. That would suggest a small animal that's capable of flight. I've seen a couple of pictures where small birds can hop from place to place, leaving behind small pits in the snow which can be mistaken for unknown footprints. I would normally rule out birds because it's winter, however this is london we're talking about, which means that some smaller birds would most likely end up nesting in buildings or close...

have you since seen what made these tracks

It lools like water dropping from the phone lines above intot the snow

A bird....i live in Quebec Canada and some bird remains in winter and they leave such tracks.... These tracks are bird tracks...obvious that in London it might look strange because you dont get much snow, but here we see these tracks all the time in our backyards.