Acceptable / realistic break pattern for 12 hour day?

I think an hour break is reasonable
Or an hour 15 mins.
But can be broken up into chunks of half an hour/15 mins if she prefers.
And yes of course should be having last break before she has to care for your child after preschool

We’ve become stuck because we always think ‘but what if I get ill and XYZ happens’. But we need to buckle up and sort it out because fundamentally it’s a huge amount of money that could be better placed elsewhere.

@Proudestmumofone1
If you do become ill or something else happens you can reassess the situation then. If you reduce the outgoings on this useless nanny you will be able to save the money and it'll be there if something does happen.
Make a decision which suits you for now and revisit if things change. Just as you are doing right now - the full time live-in nanny was appropriate until your child went to preschool and now it's not necessary.

Good luck OP!

I also think split shifts might be something to think about. Gives her chance to go to the gym, run errands, cinema, go for a walk or whatever.

I'd get two 15 minute paid breaks and an unpaid lunch of 45 minutes. She is playing you. Would you consider offering her split shifts so she works 6-10am and 3-7pm for example so can can have her down time when your DD is at school? I'd fully expect her to be doing all duties during working hours though.

She's absolutely taking the piss.
A one hour lunch break is reasonable but in that time she cooks and eats her meal, not cooking for 45 minutes beforehand and then having another hour free.
You need her to work from 4.30 to 6.15 and in that time she decides she wants to have her dinner. That's very early for dinner time. She's taking the piss there too. She's getting out of doing the very thing she is there to do by taking that as her dinner hour. It would be perfectly reasonable for her to have an afternoon break for 15 minutes or half an hour, at a time to suit you (ie. before your child comes home) and then she has her dinner in her own time after the end of her shift.
If she starts at 6.15am I think it's fair enough that she has a defined breakfast time, again at an appropriate time to suit you, not when you need her for child care. Lunch break of one hour say at 12-13. Half an hour afternoon break and then dinner after her shift is over.

She's taking the piss because she knows it's hard to find a nanny but perhaps you now need to sit down with your partner (before you meet with the nanny) and really decide what help it is that you actually need. Things have changed a lot because your child is now at preschool. Perhaps you don't need a live in nanny anymore but you do need some other kind of support with household tasks and you need childcare for specific times during the day, rather than all day.
Once you've decided with your partner what would be appropriate you can decide whether this nanny/housekeeper can fulfill that role if you redefine her role and she accepts or if other people would be more appropriate and could provide you with better support. I'd be very much inclined to try to find someone else to fulfill the role/roles you need as she sounds very difficult and shouting "no" at you when you asked her to clean a table is completely unacceptable

And after you've made the decisions with your partner, sit down with her and tell her what you have decided.

It does sound like sitting down with her and deciding on a structure for her day would be a good idea.
I'd probably expect an hour in the morning and an hour early afternoon would be reasonable, so she's there for childcare from 4.30.
If that doesn't work for you maybe you need a different person, who works say 12.30-6.30 to do some housework and then childcare after school. If you could manage to cover the morning school drop off yourselves?

Whereabouts are you? I would look into an emergency or temporary nanny and give this one notice - check the contract for notice especially given she also lives with you

It sounds like you need to redefine her role and if she won't complete the tasks you need at the times they're needed then you terminate her employment and find someone else who will. Although I don't know how you stand legally with this and employment law - I've always wondered this about nannies, because obviously they won't be needed as much when DC go to school.

A £40k salary plus board and lodging sounds like an attractive option - someone could do that job for a few years and save up a huge amount of money.

I think you need to look at what you need to happen at fixed times, what can be done whenever, as long as it gets done and then try and fit breaks around this, with some flexibility, according to her preference - so she doesn't feel she's having to have lunch at 11 am or 2.30 pm for example.

She's taking the piss. Get rid and find someone else

Although she might be taking the piss I do think you've blurred the lines with the role. Usually a nanny/housekeeper tends to the children, cooks their meals, does their washing up, might put a few loads of washing on, prep for the evening meal etc but the bulk of their role isn't cleaning.

What are you actually wanting them to do during pre school hours? If the role is predominantly becoming a cleaning role then that's not right.

The fact she has her own room.. is she a live in nanny or is she actually more like an au pair? Is she paid a nanny wage or an au pair wage?

Hmmm ok, actually in light of this, maybe consider my break schedule for your next nanny/housekeeper. This one needs to go.

When I used to work 12 hour shifts, we'd get a one hour lunch break, plus two further breaks of 25 mins for tea/snacks etc.

In your case would something like a half hour 'breakfast break' (as she starts early) work? Then a hour for lunch say 12-1, and then a short 'tea break' before your daughter gets home from school?

Start looking to replace her. She's taking the absolute piss.

It sounds like you have fallen into a pattern of her taking the absolute p*ss and you not wanting to rock the boat because you need help.

If I did anything like this in my actual job (granted, I ONLY work for 11 hours per shift....) I would be told to leave.

In a 12hr shift (NHS) we used to get
Am x 15 min break
Lunch x 30 mins
2 or 3 x 15 min afternoon breaks depending on business

Nurse working 12 hour shifts - 2 half hour meal breaks, (unpaid) plus 15 mins tea break paid. Ok a really really good day, a second 15 mins tea break!