In reply to girlymonkey:
Those were the days when the excess waiver was priced in line with what it cost to provide. Before hire companies realised that by massively inflating the price of damage waiver they could cut headline hire quotes to win business - the Ryanair model. It's still often not such a bad deal for a couple of days or so but for weekly or longer rentals the insurance waiver cost can run into hundreds.
Since then the better strategy has been to take out separate excess waiver insurance, at a small fraction of the hire companies' rates, and thereby receive the advantage of the low headline rates without the sting of the waiver charges.
The only significant downside is that you often need cash in the bank (or credit) as you're looking at potentially having to pay the higher cost of the unwaivered damage charge and reclaim it through the separate insurance company. This is often in the order of £1000 or so nowadays.