Robyn, I care for my mum. I get carer's allowance. I'm also POA for mum. If I work out the hours I spend looking after her, then my income is a pittance. Despite that I couldn't supplement my money by using her funds. I suppose I am tied by the 'rules' of POA....every penny has to be spent on mum's care.
I suppose if you started paying your mum's partner he would become an employee, technically....holiday pay, sick leave etc etc. What about the minimum wage, working time directive. A relative can 'work' 24 hours a day, 365 days a year and expect no support. Try to get an employee to do that and there are rules.
It's tricky and you need to check the rules/guidelines.
I do know that on carer's allowance you can earn £110 per week before it needs to be declared. Perhaps a fee for cutting the grass, washing the car could be agreed that wouldn't be against the rules.
I suppose ultimately as POA you can spend small sums on maintenance & who says that cannot be done by the partner. It would keep the partner happy, make them feel appreciated and not some unpaid lackey. But strictly - by the book - it may not be correct. Find out what the rules are and then see if there is any 'wiggle' room.