If you don't trust the dealer, and you're eligible to join the CSMA (Civil Service Motoring Association), they provide GAP cover from ?169 for 3 years so long as the car is not worth more than ?37.5k new.
If you take out the policy for a new car, they will pay up the difference between RRP and the insurance payout (minus your excess), so long as the total payout is not more than ?10k.
I'll apply soon within 2 weeks of vehicle delivery, and can report back on this forum.
To check eligibility to join the CSMA browse here:
http://www.csma.uk.com I use CSMA for my breakdown, car insurance and home insurance, and save a packet! [^]
Cheers, Chalky (no, I don't work for CSMA, just enjoy the benefits!)