Hi,
There was a post by RayandJax in another topic on the forum which gave a link to this website, if it's any help to you:
http://www.economycarhire.com/
We coming over in June and have also booked with Hertz but we did it through Ryanair this time and it's considerably cheaper than holidayautos, with whom we booked last year on recommendation, but you had to make the car hire booking with Ryanair at the same time as you booked the flights, as a kind of package. Last year we were with SixtIt. Here's the holidayautos link, though, in case it helps to compare:
http://www.holidayautos.co.uk/
I guess it's just a case of shopping around for the best deal, as it's quite a competitive market.
Let us know if you come up with something better as I'm sure we could all benefit from the information.
Hope this helps.
Ciao,
Frances.