You've got two separate issues here:
Firstly, you're in dispute with the insurer as to whether they should have paid out or not.
Secondly, you're in dispute with the claimant about the value of the claim.
To deal with the second issue, you're going to need the services of a solicitor. However you can try tackling the first one yourself by complaining to the Financial Ombudsman. It won't cost you a penny, so it's got to be worth a try:
https://www.financial-ombudsman.org.uk/consumers/complaints-can-help/insurance