I went home, arrived at the station my bike was parked, only to find the rear tire completely flat. I can cycle around Japan for 2 months and get only 2 flat tires, in my 1 week of cycling in London I'm already on par with that. What I have yet to find out is whether it was the godawful London roads that are to blame or if someone flattened my tire on purpose. I'll find out soon..