mdf:
What others have said about reading the policy terms and conditions.
We're with Medical Assurance. IMHO hands down the best insurer to deal with, bar none. Super friendly and helpful, always a pleasure to deal with. They are competitive on cost (though usually not the outright cheapest) but make up for it in not actually making it difficult to lodge claims etc.
But (and there's always a but) you have to apply for membership; membership is restricted to certain professions. They used to be limited to medical professionals, but have since liberalised it a bit.
One of their assessors said to us in all the time they had been assessing for them, they had never declined a claim to their knowledge even when it was the recommendation of the assessors. We have been with them for 15+ years and made plenty of claims.
I would however say, that recently their policies have become less attractive and they aren't QUITE as good to deal with as they were originally.