Can any one offer any tips/insights into GAP insurance?

My farther recently purchased a car from a dealer and is interested in obtaining insurance for any "gap" between the insurer payout and the outstanding finance in the result of an accident.

It doesn't appear that moneysupermarket offers this as a comparison service and searching on Google brings up a few options, but the policies offered by the companies found on Google don't appear to be that transparent.

One thing that we are struggling with at the moment is how to calculate how much ££ you need the policy to cover you for, for instance if you take out a £10k policy but you only have a gap of £2k you've obviously paid over the odds.

I would very much appreciate your thoughts on this

Thanks

Gareth