Thank you both. Not heard of that one hymie but it's coming up with some very good deals which I will look into, not heard of sixt either (although they seem to be sort of local to me).
Problem is I've used other comparison websites, like travelsupermarket, and the best deals are with companies I've never heard of which worries me a little.
Better than low cost hire is to be, and feel, confident wherever you drive. You can't do this using an odd-ball hire firm without multiple branches and prompt assistance wherever the wheels may fail even if in another country. Stick to the better know and reliable. I would never hire from other than well recognised firms such as Hertz, Avis, Budget and a few others. Rent-a-Wreck type shops should not even be considered. Take care - be safe where needed.
We use Eurocar (well established) and the insurance site Hymie mentioned. This means we don't have to pay the extortionate fees the hire companies want for excess CDW.
You can call them - they're very helpful and will mail you a quote (last year about £4.80 a day per driver), and the cover is more comprehensive than the hire companies' also.
I'm not advertising!
Thank you all for answers. By looking at these comparison sites and partner haggling on the phone we have got our usual company (holidayautos) to drop £80 from their quote.