Medical Certificate to dive in Spain?

I was hoping to dive in Spain in the next few weeks, but I am advised by the dive school that I have contacted that I need to produce a certified medical certificate? I have only 24 dives on my card and I have never been asked for this before. Where do I get one of these? When I have dived before I have self certified my general health and history at the dive centre, shown my PADI card and dived with no problems. I am assuming that my GP (family doctor) will need to stamp something but where do I get the right document. I am hoping someone more experienced can advise. Thanks for any help you can give.