I've heard [through talking to people at the vet's that M&S increase the level of the excess each year [or each time they pay out, can't remember which].
I've signed up my 2 13yr old cats to Animal Friends. They were the only insurance company, that would insure cats of their age. On cats over 8yrs, you have to pay the �49 excess plus 35% of the vets fees, so I have it just really for anything serious [ie: their brother has spondylosis], as my vet charges really reasonable fees.
My cover is for up to �40K per condition per year and that costs me just under a tenner a month per cat. You can pay a bit more and have life cover though. I also like the way that they aren't for profit, they give their profits to animal charities, so I don't feel so bad about using the commercial vet.
You can find out more info at: www.animalfriends.org.uk