When I had a motor bike written off a few years ago, the insurers offered a very low amount in settlement. I rejected their offer and sent them copies of adverts in Bike Trader magazine showing similar models with similar mileages, condition etc.
The insurers upped the offer to a more reasonable one.
My suggestion is to buy something like Auto Trader for comparable adverts and be prepared to haggle.
Not sure there is much between any insurance company in terms of what they want to pay out.....