Already mentioned above, but here's the links:-
Honda -
http://www.cashonda.com/offroad/intr...troduction.htm Yamaha -
http://www.yamaha-schools.co.uk/ I've done the Yamaha one - well run & a good laugh. There was a guy there that had never even ridden a motorbike before, let alone offroad. They seemed to look after him pretty well & I believe his wrist didn't take too long to heal !!!
There's also a trials day at:-
http://www.tricksinthesticks.co.uk/ The trail bike mag forum mentioned above is very good & thier magazine (TBM) is excellent.
PS. I could be wrong, but from what I understand, the Geraint Jones offroad school is for more experienced riders/ racers.