Rob
Definitely 15mm. I brought out end feed fittings with me (a tub full of various old fittings plus a few bags of screwfix jobbies) but bought the copper pipe here. The end feed fittings were a nice tight fit, as were compression isolating valves. I've just been out to check that I wasn't halucinating, the pipes are 15mm on the dot by my micrometer. I used the 15mm for new hot and cold plumbing to kitchen, bathroom and utility. My primary circuit is 22mm as it's a 90 tube system for dhw plus pool and I was concerned about having so much heat to shift.
I've seen the various even no. sizes of pipe here, I've used 18mm in the past on various bits and pieces. I agree that the Spanish are definitely lovers of iron pipe with fittings of 1/2" 3/4" 1" and so on readily available (in iron or brass). I've also noticed how much they rely on flexi couplings for coupling up everything from taps to gas water heaters, I'm not in favour of these (though I'm not a plumber), they look too restrictive (internal diameters always far less than that of the pipework they connect to, and not being compression, you still have to solder 1/2" or 3/4" brass fittings to connect to.

As far as availability of 15mm copper is concerned, Spain is a big country, ways of doing things seem to vary from provence to provence, maybe I'm lucky to have the 15mm available.
Bart,
Sorry I don't know, my cylinder is a copper thermal store, I specified 22mm connections on everything when I ordered it.
Paul