I'm with M&S at £24 per month - Greenbee and Petplan wanted in excess of £40 per month!!
However, a recent post on here mentioned that M&S were increasing a poster's premiums to a level where the insurance could actually become unaffordable. Personally, I feel I need to get the right policy at the start as changing insurer at a later date results in any known conditions being excluded - which is why you can so easily end up trapped in later years with exhorbitant premiums. Thus, whilst Animal Friends costs a little bit more per month than M&S I am going to switch. As yet I haven't heard that Animal Friends hike premiums in the later years!!