I have the same signal problem here in Lechlade. The only tip I can give, is to wander down to the local hostelry and ask what they use.
I would be a little wary if you need data services though. I tried an O2 PDA hybrid, which was a good deal, but the signal.... I tried another network with the same phone/PDA and they wanted a small fortune for it. On top of that, they also wanted another small fortune per month for a really meagre data service.
Conan
PS It is a lovely part of the World. I lived for a short time on Ditchling Common. Beer was dreadful though...