Thanks for all the valuable input. I'm trying to contact the seller to go and view the bike, so will let you know how I get on. CopperCyclist, I'm surprised that riding off is ok to do, although I do like the idea of riding off into the sunset, but if it was me, I'd want something left as security before I let anyone go off for a test ride. It would leave a very smug feeling to simply leave the seller high and dry, but then so would watching the police turn up at the door of the seller.
As I said, maybe it is all legit, but I have only ever seen one other bike like this on the road, so it is relatively rare...
Oh, and as for the CCTV footage, the police said because they couldn't see the crime being committed, and the thief had a hoodie on, it wasn't much use looking at the CCTV. I told them over the phone that I saw the thief walking in to the car park where my bike was, then 2 minutes later riding off on my bike. The camera didn't cover the place where the bike was parked, so I couldn't see him take the bike, but I would have thought that watching him ride off on my bike was pretty good evidence. To be fair, it would have been difficult to identify him with his hood up. I think it is a feature of police work in this country now that they won't/can't spend time on this sort of thing when the chance of conviction is low and the level of bureaucracy is high.