Here are some links that may help you find the best rates.
all-inclusive-car-hire.com
Do You Spain.com
Carjet
Value Cars 01933 460 888 (No website)
Concorde 0034 968 145101(No website)
Centauro 0034 902 104103
La Manga Rent-a-car 0034 968 175248
Auriga 0034 965 206400
Hertz
Avis
Europcar