Thanks Andy (x2),
I have already experienced all the extra insurances they offer, however I am fairly sure that I am covered for these by my travel insurance, but I will make sure.
The other problem I find with the car hire companies is that you book a car, say a Mondeo Station Wagon (Estate) "or similar", on the basis that they say you get can so much luggage in it, and they actually give you something like a Vauhall Insignia SW which has apalling luggage capacity by comparison. Trouble is I don't know that at the hire desk, because I don't know what a Vauxhall Insignia actually is!
From my searching so far it seems that Opodo (via carrentals.co.uk) are the cheapest.
By the way, for hiring in the UK I found these guys to be quite cheap
5th Gear Car Hire - searches for Cheap Airport Car rental UK . , they are EuropCar resellers, but much cheaper than going direct. Unfortunately they only do UK hires.
Incidentally, does anyone know how a Skoda Octavia Estate compares with a Mondeo Estate for luggage space?