Bike thieves, Gumtree, and retrieving one's possessions

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.)

Isn't the whole point of having a Brompton to be able to fold it up and take it with you, thereby eliminating the possibility of theft?

I'd offer to help you with the retrieval, except nobody would regard me as much of a threat...