I changed to AXA when Sainsbury's put their premiums up - I dont want to pay the first £125 for the girls, as that would rule out a lot of the minor claims (i.e. it cost under £100 for PEbbles to have 2 x-rays). They were very good and paid around £150ish for Mabel even though I had only had her for 5 weeks - she became ill 3 weeks after adopting her and even thought it was quite advanced cancer, they never quibbled it. You do have to watch their Direct Debit forms though, they have just had to take 3 months worth out for Zi as I forgot what they did with Molly and Mabel and didn't contact them. They do get quite a bit of money back though Gill, Molly has been insured for 2.5 years with no claims at all, they got 3 months worth of premiums for Ginger with no claims, although they got claims for Pebbles, Tiger and MAbel.
I would be wary of Petplan purely because they change the excess once they hit 9, and their payout isn't as high as AXA.