Thanks for the feedback all.  I haven't visited the Tortosa shop, only the Amposta one and the proprietor in there was a little strange (IMHO).  I've gleaned a little info from the web and was surprised to read that legally, you're not even permitted to shoot vermin with an airgun and yet after a cursory once over and a bit of typical paper form filling, you can wander around your own or anybody else's land with one of the most lethal hand weapons available and also get away with hanging up your dogs from a tree with wire when they are no longer useful.
Funny old world 'innit.
By the way, don't ask me if I've stopped beating my wife 'cos it's a loaded question.
I don't actually want a shotgun.  I just want some cartridges or black powder to help me dismantle a pile of very big and very hard olive tree roots.
Do any of you (or anyone you know) own a legal shotgun ?
Cheers again.