It won’t be that bad will it..
In 2008, when it was a little bit wet (LOL) I managed fine on Crossmarks, admittedly I was soloing so wasn’t hammering it around the course. I can only remember one short section of track where the tyres wouldn’t grip. Actually I did give up trying to ride out of the bomb holes after a while, but I don’t think I was the only one. I normally find something fast rolling works best at Thetford.
For this year my geared bike has Racing Ralphs and the singlespeed has pretty worn out 2.25 Nobby Nics. I don’t think I will bother to fit anything mud specific. If it is wet I am sure I will be ok on the Nics.