That's not what you said though. You implied that if you had your pet checked by the vet the perils covered would be lessened. That is incorrect. The policy should provide more cover if you are complying with the conditions, not less. What you seem to be referring to is that the risk is...
Did you check the renewal info properly though. I suspect a lot of people just let it renew. When they do make changes they will normally draw your attention to it in the renewal literature. If they didn't then you may well have a case. You may also have a case if you can show an annual vet...
You need to check the policy wording. If it is a condition of the policy that the pet is checked by a vet at least once a year then you're on a sticky wicket although there may be an argument if the illness is something that could have occurred irrespective of any vet visit. Without knowing...