Well, I am probably going to jinx it but so far I have found Purely Pets to be fine and have just taken out my 4th policy with them. Yes, payment can sometimes be slow and they need a bit of prodding but isn't this the case with a lot of insurance claims? Yes, there is a £99 excees and extra 15% for over 8 years and cancellation fees but these are all stated in the policy documents. There is also a 10% multi pet discount.
My advice would be phone them and ask questions before you take out the policy I have always found them very helpful.