I am with PetPlan and they don't pay out for perscription diets unfortunately.
They do pay out for food for bladder stones only.
When Moo-Moo was first diagnosed with struvite crystals back in 2007 (I think!), they paid for ALL his food for 6 months, which was very good as they say it takes 3 months to clear them up and a further 3 months to keep them away.
After he got them again when we put him back on his normal food, PetPlan would only payout for 40% this time for 6 months and the other 60% I had to pay for.........terms and conditions changed.
My vet wrote a letter to PetPlan saying that Moo needed to stay on the perscription food and they wrote back saying that they don't pay out for preventative food which it would have been by this time. They more or less said that as my vet had recommended Moo to stay on the perscription food and if he got the crystals back again they may not pay out..........so they'd got me by the short and curlies
!