It may be too late but I would propose that you pretend you want to buy the bike off him, and go to his house with a couple of your mates. Check the serial number on the bike and tell him you're taking it with you because it was yours.
This is probably illegal and dangerous etc, everyone else who posts in this thread is going to tell you that I'm an idiot and not to do this but realistically the police aren't going to do anything and neither is anyone else. You don't have to start a fight with the guy, just try and talk to him, tell him the police are already involved and hopefully he'll just pass it up. Otherwise ask your mates to jump out of the car and give you a hand. Wouldn't be a bad idea to cover your number plates etc.
Like I said, everyone will tell you that I'm an idiot and that you shouldn't take the law into your own hands, I guess it depends how much that bike is really worth to you.
Good luck.