Bawtry Paintball Fields is absolutely great - everyone I know who's been there says it's the best site they've been to. They take safety very seriously as well which can only be a good thing. Equipment is good for rental stuff and you can get a zero fog mask which helps no end - not good your mask fogging up when there's rounds flying around your head
Course wise there's all sorts - Vietnamese village, crashed helicopter, bridges and even a big tree fort to run about in.
http://www.uk-paintball.co.uk/