Hi All,
I have used the people advertised everytime I have been to Spain, even before finding this site. They are a company based in Ireland (Cartrawler) and they are just a screen scanner that looks for the best prices available.
They are very cheap and If you become a member you get an additional discount. They generally use Diesel Car who offer free child seats/booster seats too - others generally charge €2 per day for these.
In summary, very cheap and good quality service - recommend? Absolutely.
Cheers
Craig