I'm with PetPlan, which is expensive, but they are brilliant. I've claimed quite a bit over the last few years with chronic ear/skin infections and pancreatitis, and they've always paid out no questions asked. They even paid out for her to get some teeth removed at a year old, something most other insurers seem not to do.
However, as well as paying the monthly bill, apparently once your dog reaches a certain age you will have to pay out a percentage of every claim - I've yet to contact them and confirm whether this applies to us. It seems to be something a few insurers do, so keep an eye out - I wasn't aware of it (and I can't see anything in the small print) until someone mentioned it on here.