Hi --- Guessing is no good at all. !!..... You need the Vet's written confirmation of when the Dogs condition actually started in his professional opinion, helped by his records.
If that date is after the date the policy started from then you have a perfectly valid claim to put forward to E & L insurance, it's as simple as that, they cannot contradict the Vets professional view and opinion simply by GUESSING ?
Go back to the Vet for this report, even if it costs you can reclaim that cost too under the insurance, seeing as they have forced you to incur extra costs.
If they still refuse after that to honour the valid claim, take your complaint to the Insurance Ombudsman at the FOS, below.
http://www.financial-ombudsman.org.uk/