Some useful links for you:

http://www.paynplay.co.uk/index.php?...se=4x4%20tyres

http://www.4x4calendar.co.uk/

Try and find a club local to you, their promotion/drive days tend to be better run and organised.
Unfortunately some of these sites tend to just let any fucker in with an EBAY special. Some people i know went to a well known site run by experienced people and there was little marshalling and people were going up hills when people were coming down etc....

Also register on here and see whats going on near you:
http://forum.difflock.com/ Its a good site, you just have to ignore some of the jobsworths!