I hear you about Goldcars and sorry to hear about your experiences with them. Although I haven't had a bad experience (fuel, allegations of damage, and so on) I will certainly think twice the next time. I will admit to going around the car and videoing it when I handed the car back in just in case, and have been watching my credit card like a hawk just in case.
On the positive side, and for the future, is there any in Spain that you would certainly recommend based on something that happened and them doing "the right thing" with little or no fuss?