This was batted around another thread a little while ago - I'm assuming you're familiar with the ins and outs of what you're looking for in a GAP insurance policy.
The one I went with in the end was http://www.car2cover.co.uk/
who worked out at £210 for 3 years up to £25k payout. My Audi dealer quoted me £495 and tried to persuade me that theirs was a good deal as it was transferable to another new car if it was written off in the first 6 months. Sounds like a good idea, but in that case you would have another new car (paid for by your insurance company, in all likelihood) but you'd only have 2+1/2 years of GAP insurance left on your 3 year policy.
If you went down the route I did, then yes, the policy is not transferable to the new car. However, just go and buy a whole new policy and you then have 3 years of GAP cover on your second car (an extra 6 months) AND you're still £75 better off!
One thing to watch out for is what the GAP insurance company uses as a valuation. Some will use the Parker's top valuation at the time you buy your policy - generally insurance companies value well below this, which leaves you with either a big gap (kind of defeats the object) or else with a dispute between 2 insurance companies with you stuck in the middle. Some will also not cover any optional extras (only list price of basic car) and others won't cover the VAT you paid to Gordon Brown and his cronies...
The one I've used is actually through Hitachi Insurance (big company, so feel comfortable they're not going to go bust or disappear) and they also insure back to the original sales invoice value for new cars. They only apply a 110% of Glass's guide price if you buy a policy for a car of over 120 days old, so you shouldn't get into a dispute with them (hopefully - never any guarantees with insurance companies!