Chris spotted the ad on gumtree yesterday and told me. I instantly updated the police report. They said they'll call back but they didn't. After the ad was pulled I decided not to wait and arranged to meet the guy with the intention of going there with the police. That was according to advice from Andy and that thread he linked to.
I called up today around 1pm and told them what happened. They said they'll call me and... they did, at about 4.30pm. But this time it was a pretty cool officer who asked me to email him a description, photos, and the saved ad page. They handled it from there. Basically two of them went in plaincloths, using my description as a crash course in bike components they bluffed their way to haggle the price down to £150 (!). According to one of them he ended it by saying "how about I give you this much..." opening his wallet and showing his ID. Fuck yeah CSI! They called me straight afterwards and I just went to the station, filled a 5xA4 page statement, and walked out with the Surly.
So thanks to Andy's advice, the two coppers' help, and especially thanks to Chris's detective skillz, we got the bike back.
From the manc thread, Yay!