Thanks for your comments guys - some interesting thoughts, and definitely things to ponder on.
I think to be honest, one of the things that made me decide upon the action I took was when my wife told me she'd decided not to call the police, because of the 'upset' it would cause over such a minor hit... like many of you, I can see a future problem waiting to rear it's ugly head with him, and was in no mood to allow him to abuse my wife, no matter how minor. If it was purely revenge, I would have done far more damage than I did, and since that wasn't my intent, I used 'enough' force to punish, subdue and warn him of worse to come should he hurt my wife, or his mum, again.
I feel 100 percent justified in my actions - If I get arrested for them, so be it. I have already spoken with the police, and they are very aware of his past violent tendencies, and other issues too.
SixofNine wrote that if he had done something similar, you wouldn't read about it here. I think this is the perfect place to discuss it - I can't PM everyone, and wanted to know other peoples reactions and feelings, and the only way I can do this and get a frank and honest opinion, is here. My shoulders are broad enough for criticism, and I certainly didn't post in the hope of praise for my actions - I personally don't feel it was a 'smart' thing to do, but I do feel it was necessary. BTW, he has a personality disorder, he isn't mentally challenged. He knows EXACTLY what he is doing, but takes no personal responsibility.
jwbot - that's an excellent suggestion, one that I'm seriously considering. I doubt that would hurt too much at the minute, as I guess I'm not welcome anyway!! Funny thing is, my wife and her sister had only been discussing his mood swings earlier that day, and his mum suggested that he should be on medication to 'even out' his temperament. She knows she has a problem with him, but just isn't prepared to swallow the bitter pill that comes with the admission that he is far from 'stable'. You can't help those that won't help themselves, but you can certainly make sure they're aware of the consequences should they vent their anger towards my family again!!!
Thanks again everyone for your frankness.
Bull!