We recently changed to M & S - if you go back through previously posted topics, you will find a discussion that I posted in the first half of September ...
Basically, look for lifetime cover (ie if your dog develops a complaint that will last longer than one twelve-month cover of your insurance, the Insurer will not disqualify that from the subsequent year's cover etc ... (ie if it should develop a condition in month 11 of your insurance, they will refuse to cover you when you want to renew your insurance..)
Happy hunting !!