Agree with Gill on the Axa recommendation.
The main thing to look out for with pet insurance is that it's true 'life cover', and by that I mean that they will pay out for a condition for the life of your pet, not just for the first 12 months of the condition. Policies that offer X amount per year with the 'pot' reset each and every year are the best type and offer true life cover. Some policies describe themselves as offering 'life cover' but they only offer X amount per condition and what with the technological advances in veterinary medicine you could find your 5k being used up very quickly and then you wouldn't be able to claim for that condition any longer.
As far as I'm aware the companies that offer true life cover (ie X amount per year, each and every year) are:
Axa, Petplan (not the value policy), NFU, HSBC & Halifax.
Axa are the best (imo) for a few reasons - they are underwritten by themselves so no danger of changing underwriters and therefore less chance of the T&C's changing drastically as a result, as happened to M&S a few years back. They have a high level of cover (7k each and every year), they don't charge a % excess for older pets (most insurers charge 15-20% excess for pets over 8 yrs old) just a flat excess of £75. Their premiums are reasonable and so far *touch wood* the increases at renewal time are minimal.