First of all, you need to get lasting powers of attorney in place if your mother is capable of understanding what she's signing (that's if you haven't already done so). If not, go for the more onerous deputyship.
My mum lives with me in my house. She pays half of food bills, petrol, gas, electric and water, her own clothes and speficific toiletries. She also pays for her twice a week carers, day club, chiropodist etc. If she's been away or has had one of her don't want to eat much weeks, I reduce her share of the food bill to reflect that and never put alcoholic drinks on there. I pay the phone bill and all other car running expenses in their entirity. Council tax is not payable for her as she has mental impairment, or for me as I'm her carer and not a spouse or under 18 or something else I can't remember so I pay what's left i.e. 50% I also pay all maintenance on the house and for capital type expenditure, like a new tv, washing machine, fridge etc. She tells me she should pay towards things like that as she has use of them, but I'm not comfortable with that, so I say thanks Mum and don't bill it - she is beyond checking things like that and forgets anyway. She paid for the stairlift and rails round the place that were put in specifically for her (and I find a nuisance, frankly) .
I use one credit card for most joint expenditure and split the bill monthly. I also keep a notebook in the kitchen and jot down cash expenditure and split that (don't always botrher for small amounts tho'). Once I've worked out her "monthly bill", I do a transfer from her bank account to mine. Where something is specifically for her, I pay for it out of her bank account.
It all sounds a bit complicated, but a socliitor from whom I sought advice said this was all fine. Once the system is working, it's a pain, but easy enough to keep going. I do keep full records and all receipts. Solicitor said Mum e could also pay towards things like the white goods/tv etc as she had use of them. Given they will remain here if she goes into a home or when she dies, I felt it would save messing about with famliy if they choose to make remarks like well Mum paid towards this that and the other and you've still got them.