In the search for my stolen bike I was told that the following are the best places to look. Unfortunately, having looked, I have to agree - lots of suspiciously cheap bicycles of dubious ownership.
Brick lane market (Sundays from ~04:30 but most stuff won't be there until ~08:00)
gumtree - turns out that it's a hotbed of dodgy goods
london second hand shops
car boot sales - check your local free-ad pages/newspapers
Some (a very small minority) bike shops in London - not going to name and shame as I have no proof.