A second vote for Zen – customer service as it should be. Switched to them when we moved house solely on the reports of good customer service – as wwaswas says, out in the sticks distance from the exchange and quality of lines is likely to be the limiting factor, whichever ISP you choose, but we'd had such s**t service from Pipex we were willing to pay extra just to get better customer service.
After a flying start for 6 months (faster and more stable than we'd had in that London), we started getting a lot of trouble with our connection just dropping. Zen were excellent, and actually ended up contacting BT direct on our behalf, and were willing to swallow the callout charge if it turned out not to be a BT problem (it was – the little grey box on the nearest telegraph pole was full of water…).
Couldn't recommend them highly enough.