'These are everywhere in different colours and usually cited as Mdina, but I'm pretty sure they're not. They're new I think' - in view of this response, I immediately gave it back to the charity shop, thinking it was perhaps Chinese and of no interest. Huh.
I'm not being serious - and in fact in view of the quite massive crack it almost certainly would not have been a keeper. However, the important thing is that I will now recognize this type of bubbled colourway, and the style of snapped pontil, when I see it again, and that is the really useful part. So thanks to both of you.