Probably teaching your grandmother to suck eggs (which is the oddest phrase ever!), but have you made it crystal clear to FoS that this is a very rare car and that your insurer are treating like a normal one?
Who are you with, we can look up the wording online no doubt and check what it says,
You shouldnt have to buy extra insurance (gap) to cover what is supposed to be insured. Okay, they always haggle over a few quid but not a few thousand or 10%s of the car's value.
NAME AND SHAME if it fails as I dont want to be insured with the same people.
Earlier someone mentioned an exige; my elise is covered by a specialist insurer on an lotus scheme with track time included, so you know they know it's a hard car to value and that after 8 years its lost 30% not 75%.
People do need to be careful with cheap insurance, or anything in life that's cheap. It's cheap for a reason. A friend found they had a £1,000 excess on water escape in the home whch is one of the most common causes of a claim, meaning it more or less wasnt insured. If they'd known, they would happily have paid more for a "proper" policy. Not saying OP has cheap insurance, but just be careful.