Bike thieves, Gumtree, and retrieving one's possessions

bring a photo of your bike for further evidence, ask him question about the bike that only you know (either way the 'seller' may not know even the saddle make).

Unfortunately it's a pretty standard Brompton, though the spec matches exactly. (I have warned him that about 3,658 Bromptons get stolen and sold in London every day, and that this is almost definitely stolen, but might not be his - but he seems convinced. Anyway, he's got his name written all over it in UV and visible permanent marker, so it shouldn't be that hard to identify.)