[Ref: policy number]
Thank you for your letter of [date], in which you offered me [£…] in respect of the damage [describe] to my vehicle [make, model, engine capacity], as a result of [circumstances causing damage] on [date].
Since receiving your offer I have obtained an independent valuation of my car from [organisation/garage] to the effect that the market value of the car at the time of the above incident was [£…].
A copy of the valuation report is enclosed.
In light of this evidence, I am not prepared to accept your offer, and I expect you to reconsider it, taking the above valuation into consideration.
I look forward to hearing from you within 14 days.