thanks for the advice, it's all very helpful. We've got a booking with Alamo, since they're recommended by Guardian/Which magazine as transparent about fuel charges, although I've also been reading about long queues.
Vanrell comes up a lot over on TripAdvisor forums as a good local alternative-- clear about no fuel scam, and on a different floor of the garage, so no queues. Anyone with experience of them? I could get an automatic for the same price as a manual with Alamo, which might be useful on mountainous roads.
(and just say if I should move this to the Spain forum, I had a look but there doesn't seem to be much activity lately)