If you are 100% sure it's this guy who has the bike. I mean 100% SURE and the police won't do anything. Then it's time to get physical i'm affraid.
It's won't be hard to 'attempt' to batter someone if they have your bike, im not saying you will beat the guy or whatever because i don't even know you.
But atleast f'in try, i wouldn't even bother getting friends/family involved if i was gonna' hit someone to get my OWN bike back. Too much shit for them to be honest, get a friend to talk to him about it, he arranges the meet, YOU go to the meet, as soon as you see him holding the bike, literally run up to him and smash him as hard as you can...He will probably fall down and you've got your bike