Thetford is riding lovely at the moment, a little sandy but quite fast. Gets very muddy in the winter and after lots of rain, but still good fun on a singlespeed.
The black is good at the moment, not a true black, its not very technical at all. The red is very good, with the first technical sections in the forest put in by TIMBER, the local trail group. The forestry have been down lots of stone, which is loose in places, but getting better.
There is miles and miles of lovely flowing singletrack but does take some exploring to find. Oh and there are lots of bombholes which are very fun!
Park at Brandon Country Park not High Lodge as it free but can get busy!
Check out the forum at http://www.timbermtb.org for more info
If you do go, enjoy!