Love the replies here, stop talking shit, like any of you'd ever be retarded enough to commit GBH with intent....you'd just f*ck yourselves over more than what the guy has already done to this poor lad's car. When have you ever heard of "justice done" in this country?! You're simply digging your own graves.
If there's no proof or witnesses, there's nothing you can do. It's horrible to say, but you have to move on.