Dear [name of insurance company]
[Ref: policy number]
Thank you for your letter of [date], in which you rejected my claim against the above policy for [damage] on the grounds that [reason].
I refute your argument on the basis of the wording of the policy, [quote policy wording].
Furthermore, I enclose a report from a relevant expert, backing up my claim as follows [details of report].
This shows that the damage was caused by [cause], and was not in any way due to [neglect etc] that you suggest.
I am therefore covered by the terms of this policy, and expect you to reimburse me the sum of [£…] as detailed in my original claim of [date], within 14 days.