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Are the races usually held on the established trails there? ( Limeburner & Beater ) or are things mixed up a bit?
Not sure I'd fancy the Bracken Pit on a CX bike. Everything else would be ok.
Usually a mixture of the existing marked trails and some other unmarked bits in my experience. That pit is definitely doable on a CX bike - I've seen it done. Admittedly by someone with far more experience on a CX bike than me. It's easier now they've put those rocks in place than it was before, when it was just a wall of sandy soil you had to scrabble up. With those rocks in place it's really just a question of carrying enough momentum to take you up and over the top.
BareNecessities

Brommers
IR
Is this for the Kings Lynn MTB winter series? If so, which one(s) are you doing?
Edit: Successfully stalked you. Round 3. Hoping to be there myself. Same race, different category.
Can't help on gearing I'm afraid. I ride at Thetford quite a bit, but always with gears. So far. I wouldn't worry about a short stem for handling technical sections though, because there won't be any. Every race I've done at Thetford so far would be perfectly doable on a CX bike.