I think the company cc1 is thinking of is ‘insurance4carhire.com’
To be honest such an insurance is only worth considering if you take out their annual policy (and hire cars abroad regularly throughout the year).
Should you take out such insurance at the time of collecting the car – the insurance will probably be with them (insurance4carhire.com), but with an added cost for the hire company’s cut.
My advice is to carefully check the cars’ condition on collection – check that any damage is recorded within the hire documentation – if not, return to the hire desk and ask for a vehicle inspection to record the damage.
Take photos of any damage. On returning the car, make sure that you get the paperwork signed off by the hire company's employee, showing no damage during your hire period.
Also take photos of the car on return.
Since the hire company will have your credit card details to charge any damage against, should you follow the above advice – you will be able to challenge any unjustified charges.
I would concur with cc1, that Carhire3000 offer competitive prices.