Second-hand bikes in London

Apart from eBay and here is there anywhere else I can look? I see you guys think that gumtree is a bit iffy, but I'm really hoping to pick one up for around £40 (complete) and I don't mind if it needs some tlc.

Given this is London, the majority of bicycle around that price point will likely to be stolen.

car boot sales is a good place to find cheap bike.