Thanks for the replies; I now have a clearer idea how to deal with the diagnosis question and how much to tell her. I think her condition is too advanced for her to comprehend and I'm pretty sure that, as witzend's anecdote illustrates, she would not accept that there was anything wrong. For quite a while she has been living in a fantasy world where hard facts are meaningless to her and what she wants overrides considerations of practicability. So I will apply the principle of "love lies": thanks for that, Chemmy. She has often said "I'm not senile, you know" and I'd imagine that to contradict her would give birth to all kinds of monsters. Raggedrobin: I also would like to know if I had this illness, but only if I were in the initial stages and could process the information with a measure of rationality. I think my mother is too far gone and it would probably makes things more difficult, both for her and for me. So, thanks everyone for helping me make a decision.
RedLou:
My tentative investigations have not uncovered many English care homes on the Costa de Sol; I've sent an email to one and am awaiting their reply as to fees and availability. It's quite surprising as I'm sure there must be other people in a similar situation. I'll have to dig deeper. The good thing about the care home she's in is that it's under the aegis of the local health authority who are able to offer assistance with the fees. It would be wonderful if I could find an English-speaking establishment which operates under the same conditions but I'm not entirely hopeful that such a place exists. I've pretty much ruled out a return to the UK, for the reasons stated further up the thread. As for friends, well, she doesn't really have any apart from the two I mentioned; sadly, she drove the rest away. The stealing accusations are terrible. It's absolutely impossible to persuade her that they are unfounded. All the nurses in the care home have been stealing from her even when they're nothing to steal! I now understand this is one of the most common symptoms; if only I'd known before. Actually, if I hadn't been in some state of denial I could have sought help earlier, but we are where we are and can only move forward.
Owly:
Thanks for the EHIC card info, although it may be academic now as I've pretty much abandoned my plan to bring her back to the UK. As for selling the house without POA, this is something I need to look into urgently. I can draw on the expertise of a couple of Spanish lawyers who may be able to find a way around this; one of them suggested selling my mother's half of the house to me, which would grant me full ownership and hence the right to sell. However, I don't understand how this would work legally if she is non compos mentis, although it sounds promising.
If anyone thinking of retiring to a foreign country is reading this, let my situation serve as a warning; please get your affairs in order long before it reaches this sorry stage.