Thanks for the advice so far, I must say I am rather surprised at how busy some gliding airfields can get. I imagine the emphasis on maintaining a good lookout would be useful when flying powered types as well, particularly to fly-ins etc.
I would probably try and stay with one club if practically possible, preferably a place which has a reasonably large membership as from what I can tell it should be easier to fly often. I suppose one of the most important things is the ratio of gliders and instructors to students. That said, other facilities (such as a canteen) would be useful.