Hey Pez,
I live just up the road in Ipswich so ride in Thetford often. The grading does NOT compare with 7Stanes!!! I am quite a novice and struggled with Kirroughtree blue trail, however completed Thetford red the other day without too much difficulty, wouldn't have dreamed of trying a 7Stanes red. People like thetford because it is fast to ride as it is so flat, there is a new section in the red with a few jumps and a skinny but as a whole the trail is worn out through over riding. Nevertheless, I love it! I haven't done black but my OH has several times, he enjoys it, but again it is very eroded - the black one does have bomb holes to play in though which my OH likes. Both routes are similar length (16/17km) but the red has a shortcut.
Be warned BTW it is £6/7 to park at high lodge, if you go 2 miles up the road and park at Brandon country park it's free. Have fun 