Best tyres for me in that part of Wales is Eskars, the new ones coming out are even better. Tried Fat Alberts, Alberts, High Rollers, Nevegal+BG. Advantage Crossmarks. I found crossmark on the back around Brechfa, Cilycwm & Doethie Valley lethal in the wet. The trails will be wet, will be probably be the same with the weather here in the last 4 weeks and no sign of let up for the next 2 weeks.
If you want to get another reccomendation, try Nobby Nic up front and Racing Ralp in the back and get to know them. I have swamp things for winter.