Go onto http://pitbike.forumwise.com/forum.html theres everything you need to know and its all a forum for pitbikes bit like this forum but for pitbikes. There is a nice track down in Dartford.
I would highly reccomend the bulldog 125, i've had one. They are fast as f*ck and so torquey it's unreal. Godd strong reliable bikes aswell, also all the parts are level 1 entry race spec.