I think premiums tend to go up in relation to total claims to the company rather than or as well as individual claims, as well as increasing due to age. Jaffa's premium with petplan certainly went up a bit each year even though I didn't put in a claim until last year when he was 9 years old. Companies are always looking at ways to recalculate the way they work out premiums and they can go up or down depending on how they do it.
I think that you need to read the small print carefully re the no excess with M&S. If you had no excess before you should still have it, but I think others on here in the past have had problems with the actual policy wording not being clear and have had to ring them up and insist on new documents being sent that make it clear there is no excess.