Thank you to all of you who responded to my cry for help. I agree, I have been far too patient, you all seem agree that I have to find a better specimen and maybe try a better location. I'm not so sure if I'm able to find a better spot, though. I wasn't quite correct when I wrote that it was not irrigated,except for the showers we take outside in the summer; the tree has actually been on an automatic drip system for quite a few years, but the small, red, plastic sprinklers tend to get clogged with calcium deposits and they need to be cleaned frequently, which is not so easy when we are not there.
I suppose I was unlucky with the tree we chose, I'll see if I can find a better one when we are back in the autumn.