Advice Needed With Insurance Company Refusing To Pay Claim

Hi,
Looking for some advice about a pet insurance claim:-

Some time in September 2018 we noticed that our pet dog was licking his front left paw continuously, on examination we discovered a small hard lump that must have been irritating the dog to cause him to lick it.
Took him to the vet who advised that it could be surgically removed to stop it irritating the dog.
Prior to deciding on surgery we applied to the insurance company for pre-authoristion which they promptly declined for the following reasons:-

1, Quoting from the vets records:-

13/05/15:- "Lump on LFL dorsum metacarpal area, small size hair growing from it looks as wart not attached to bone"

17/03/18:- "Several soft lumps all over body."

They concluded:- "The lump is a pre-existing condition from May 2015 therefore the 365 day limit has been exceeded.

2. And:- As you have taken out a non-lifetime policy we are required to place an exclusion clause on your policy with respect to cysts and warts effective from 1st May 2015.

The lump continued to be an irritation over the next week and grew in size so we decided to have it removed. The surgery was carried out successfully and the dog made a full recovery.

We decided to resubmit a claim for the vets fees and wrote a rebuttal of their reasons for non payment as follows:-

1. Proper examination of the vets notes show that this was not a pre-existing condition. the lump was not present in any examinations after 13/05/15, neither was it found in an examination on 26/05/18 which is 26 days after the policy was renewed.

2. After the surgery we had the pathology test on the lump.The results show that the lump is a "fibroadnexal hamaratoma which by clinical definition and histology is neither a wart or a cyst.

The insurer responded asking for the date the lump first appeared.
We don't have a date all we can say, using the vets notes, is that it started to grow between 26/05/18 and the date the vet advised surgery, a window of just over 3 months .

In response to that information the insurer has now written asking for an exact date we noticed the lump.

It seems that the insurer is just trying to avoid paying the claim.

It is obvious from the vets notes that this is not a pre-existing condition
How can they apply a retrospective exclusion to the policy.
The pathological exam showed that the lump was not a wart or cyst so the exclusion would not apply
Do they really need an exact date for when the lump appeared when the vets notes clearly show that it appeared at least 26 days after the 2018 renewal date.

We are quite prepared to take this to the ombudsman if needs be but any advice from the forum members would be appreciated.

Thanks in advance.