When the different policy wording was issued to you, was the change in cover pointed out to you, as the Insurer has a duty to bring changes to your attention.
But if the condition of visiting the vet on a regular basis is not in the policy wording, then as a retail policy, the Insurer has not provided a policy wording that is clear and therefore one that the FOS should be aware. Of course the Insurer could always pay the claim, if they know that they...
What does your policy wording say about vet visits. If the Insurance company are refusing to pay the claim, I assume that your policy either has an exclusion or a condition that mentions the need to visit the vet at regular intervals, and the Insurance Company has referred to this when declining...