probably not much help as i'm not sure how close they are to cambridge but there is skirmish at necton which does walk-ons, as well as one on plumstead road in norwich (pretty close to the train station), i think there is also a site at thetford (all action paintball rings a bell but that could just be me) and i think they put up a sup air site at snetterton or somewhere...those are what i remember from when i was living in norwich so one might be in your direction or whatever as it's only an hour away or so.
hope that helps
Lee