Well .... sorry, but beg to differ.
If your insurance premium was for example £2000 , but you've only paid them £500 of it, a quarter, they would quite rightly in my opinion, only offer you a quarter of what the car is valued at.
Obviously I don't know the exact figures, but you can't expect a full payout if you've only paid a third of the premium. Clearly though as others have said, you need to get as much as you can. Hunt around the classifieds for a similar car to yours and quote them in any corres with the company.
Sorry that you've done the car in though mate.