Her working hours should be set out in her contract of employment.
If no formal contract previously existed, one can be assumed from the conditions of employment under which she has worked until now. These conditions can’t be unilaterally altered without her agreement.
In terms of public (as distinct from bank) holidays, the legal position is outlined here
You can’t read that extract in isolation of the rest of the provision.
It all depends on the specifics of the contract of employment or, in the absence of a formal contract, the implied specifics of the contract I.e. how things have been done until now
ah ok, they used to get paid by me fifth of their week if they were not rostered to work that day so that should continue .
I am pushing for her to ask for a contract to formalise things. Hopefully will get one soon.