Ideally you want a strong, lightweight rim. Ideally 32 hole with thick sidewalls(lightweight, holds grinds / doesn't dent easily ) . Make sure you have a nice tight build though, preferably with brass nipples due to the high amounts of truing that can come with riding trials. To be honest you're best off getting either a try-all or echo rear rim, the using pretty much anything light over 25mm wide on the front. As for tyres id say minimum 2.00" on the front; 2.3" on the back...maximum, 2.35" front, 2.5" rear. Try talking to Trisandhisbt on here, he's from the Northants. I used to live there as well, and ill be up for a week or so at Easter.