Firstly, never accept the first offer, ever.
Next, gather evidence to show how much you spent, what you paid for it, other similar car valuations and classified and eBay links. If you do their job for them they will find it difficult to resist an increased offer. What are you hoping for by the way ?
I've been through all this with an old VW and ended up not only stripping the car of all expensive parts but making a profit on what remained. I highlighted the fact that I'd saved them a fortune in storage costs because I managed to get the car to my parents driveway myself (it still drove fine, testament to VW build strength) and that I was car-less and had to hire my own car (the courtesy car only lasted 2 weeks).
It took 18 months before it was finally sorted but like I said, I did all their work for them including computer simulations of what actually happened in my case. I had photos at the scene and even got a witness.
Are you going to buy it back, restore it or use it for parts ? Just check out sites like
www.carandclassic.co.uk &
www.classiccarsforsale.co.uk for a price guide.