Would the difference in price between between what the insurance company reckon it is worth and what the trade reckon it is worth, ever be as much as £10K? I reckon £7.5k would cover it no problem, possibly even 5?
If i understand your reply properly i think you may have misunderstood what i tried to explain.
As an example
Say you wrote your brand new car you just bought now off in 4 years time.
The insurance company pay you 'Trade value'. This is normally similar to the 'trade in value' in glases guide or parkers etc. This in the case of a GTI maybe only around £10K in 4 years time.
If you had 'back to invoice' and you paid £22K for your car, the gap would pay the £12K difference.
It all depends how long you may keep your car and the term you get the gap for.
EDIT: Im sorry i just reread your post i misunderstood what you ment. DOH!