There are 2 types of GAP insurance - RTI (Return to invoice) which is what TIFF-C20WTH was offered, and Finance Gap - which is what i think Damo V is on about.

The RTI Gap is generally a very good policy but if you need to claim on it there will be a limited time between your car being written off and you submitting all the relevent paperwork to the insurers for a claim.

I work in a large car supermarket and we sell both - I would personally say that they are worth the money. (we would charge between £199 - £999 depending on how deep your pockets look!)