We are with M&S too and have just had two massive price hikes... Our 6-year old lurcher went from about £14 to £25, and our 8-year old pyrenees went from £29 to £54...
But unfortunately all our dogs have had claims, so we have to stick with them. I must admit though, it would be difficult to get anything cheaper for our pyrenees anyway and they have never caused any problems with the claims we have had.
Vera