As far as tyres go, all the lads I ride with in Plymouth really rate Bontrager mud-x for wet stuff. The mud down that way doesn't get too sticky, although 1000 riders over the same bit of dirt for 24hours may change this.
If it dries out, I'll be running Maxxis Crossmarks, stays damp and I'll be sticking to the Kenda nevegals I've got on at the minute. If it gets nasty, might think about a proper mud tyre.
All the climbs from last year were on pretty solid ground, so it's only for the descents that you'll need super-grip.