NorthBerks MCC are an excellent club, they have some excellent club trials and also run the Wallace Cup which attracts the top UK riders and a fantastic arena trial, built by Stuart Robinson and the club members. The sections in the arena, are mostly man made, very spectacular, watching the likes of Dabil and Brown ride there though. If you've ever seen indoor trials, then the sections are very similar in the way they have to be ridden, but outside. They also have practice days there too, £6 for the day if your a member.
I would recommend you join a few clubs, gives you plenty of choice on where you can ride and the type of terrain. When I was riding, I was a member of Oxford ixion, NBMcc, Blackmore Vale, and a few others. There are a fair few clubs in your area, that have some excellent trials. If you want to get in to trials on the cheap, then get a 2 stroke, 4 stroke is my passion, but am only dreaming at the mo, as I haven't ridden for a few years,(a 1989 Honda RTL 250 Rothmans is on my wish list, very rare though)just observe for the club now. GasGas are very popular and then I'd say Beta in this area, but the best built, most reliable, are Montesa, as JB has said. There built in Spain, like most trials bikes, but are owned by Honda, with Honda engines. Used Bikes in the south tend to be less bashed about, than those from the north, as the sections down hear are mainly mud and tree roots on banks and hills, where as up north they have a lot of rocks and stream sections.
Buy trials & motox news paper from the news agents, it's weekly paper and has lots of info on club trials regs, events, dealers and a classified section for used trial bikes. You can look up for trials regs for a particular weekend and enter a trial in your area, or travel further afield. A lot of clubs have a practice area and there's plenty of friendly advice on hand.
Don't rush out and buy a bike, go to a few trials, watch and ask a few questions about what you can get for your money and have a go on all the makes of bike your interested in, there all pretty good these days, but all feel very different. BVM in Stroud have an excellent reputation and use to be an official BM Dealer, very nice people, bit pricey, but excellent service. Don't buy anything really old or obscure, as you'll almost certainly have lots of problems and hassle.
I know your pretty handy on a road bike, but riding trials will improve your skill, i.e throttle control and balance big time, most road racers ride trials and a lot of the best enduro Riders are ex trials riders.