If you have reached deadlock with your insurer, you can refer your complaint to the Financial Ombudsman Service (FOS). Use the Which? template letter to do so.
[Ref: insurance company and policy number]
I am in dispute with the above insurance company which I understand is a member of your scheme.
I have been unable to reach a settlement with the insurance company in respect of an offer it made in connection with [item] following my claim of [date].
I am therefore referring the matter to you for investigation.
I enclose copies of all relevant correspondence, and look forward to hearing from you in due course.