I had my car stolen and my insurance company sent me a claim form asking me to put a value on the vehicle. I put down a figure of £1,000, but later discovered it was worth a lot more. However the insurance company says it will only pay out on my original £1,000 estimate.
Its up to you to provide evidence of the losses you have suffered. If you havent cashed a cheque from the insurers in settlement of your claim you should be able to persuade them to raise their offer if you can find evidence that cars similar to your own are currently selling for more than £1,000. The small ads in the paper are a good starting point, or you can quickly find the book value online.