Thanks everyone for your replies. I know in my mind what I need to do both now and in the future.
I see that there's no point in trying for an LPA because mum wouldn't understand what she'd be agreeing to. Social Services are already involved and we get a visit every 10 days or so. The frustrating thing is Mum's on her best behaviour when they're about. I don't think Mum's ready for a care home yet, we get by, just about. She's physically capable of fending for herself, it's just that she doesn't, although at the moment it's mainly silly things. She won't eat properly, if she had her way, she'd live on cakes and biscuits. I see her daily, and most of the time prepare and cook her main meal. I've given up trying to get her to eat fruit, it just sits in the fruit bowl looking sad. Part of me is of the opinion, it doesn't matter what she eats, as long as she eats something - but then I have to listen to the graphic details of her constipation, although she takes laxatives as if they're going out of fashion. I don't know how I can deal with that. Mum's very secretive about stuff, and I only find out by accident things like this.
Mum's not aware that she has dementia, which can also make things more difficult. At the moment, I'm not sure whether to tell her (on a day when she's capable of listening) or not. That's something I will have to discuss with my brothers too. Although I'm not convinced she'd believe me if I did tell her.
I've just spent a couple of hours with her, watching her stare into space, and sigh every time I tried to speak to her, getting monosylabic answers, and stares at me as if she's saying "who do you think you are prying into my life". It's so difficult not to react.
I shall be speaking to the COP about registering the EPA, although as I'm a signatory on her bank account, I don't think it will make any difference, but it's best to have these things done properly.