If I were you I'd call 999 before you head over and tell them that you fear the criminal will get violent and that they should send an officer.
I'd also be tempted to just ask for a test ride, ride it off, and then have group of mates move in to surround and detain the criminal until the police arrive. That way you will have your bike back already when they realise they've been trapped. Less risk they will damage the bike or anything like that.
If I were you I'd call 999 before you head over and tell them that you fear the criminal will get violent and that they should send an officer.
I'd also be tempted to just ask for a test ride, ride it off, and then have group of mates move in to surround and detain the criminal until the police arrive. That way you will have your bike back already when they realise they've been trapped. Less risk they will damage the bike or anything like that.