I used to live in Cambridge and very vaguely recall a Mountain Bike Club called something like "the mutts nuts treadley club" .
They might be worth trying to track down. Mike's Bikes in Cambridge used to have some contacts there I believe..
Other than that, get yourself over to Brandon and ride the Black run at Thetford Forest! If you ask at the little cycle hire place they'll probably put you in contact with some clubs etc..