Never accept the first offer!!
Have you any receipts that show you have added money to the cars value, or kept it in a first class condition. If so take photocopies.
Have a look at ebay, gumtree and look for the same year, mileage mods etc and take a copy, hopefully will be a higher price as well.
Telephone the claim handler, if you can, as its always better to get to know them and explain why its worth more. Send off the copies of receipts Photos of the car and copies of the higher value adverts.
Good luck, with it.