Peb,
Sorry, I don't know the exact details. I do know that Spanish law requires you to carry a Spanish translation of your insurance certificate. As far as I know, most (maybe even all) UK insurance companies issue a Spanish translation of their certificates in any case, but I'd suggest talking to your insurance company if you want to know more. (Alternatively, I'd guess that the lack of replies might mean that no one on this forum knows the answer, but there's bound to be someone on the Private Flying forum who's flown to Spain who would know.)
FFF
--------------