Thetford Forest is certainly your best starting point, and there’s some truly excellent trails there. After that, you’re at the mercy of local bridleways, perhaps stick your nose in your nearest shop to see if anyone can recommend anything. Remember that the Chilterns aren’t that much further than Thetford and there’s stacks of top-notch riding there. Perhaps grab a copy of Mountain Bike Rides in and around the Chilterns by MaxDarkins pb www.roughrideguide.co.uk. And of course, keep checking the forum.