"Free" 15 hours childcare

Hi, can anyone advise please?

Our son and his partner have recently been told by their daughter's nursery school that they have to pay top up fees for her free 15 hours a week place there. Apparently they forgot to charge for the last academic year but are now trying to claim it for this year (i.e from September 2018) at a rate of about £34 a week which their invoices say is the difference between their fees and the funding they get from the council.

I have looked online for them, and the Gov.uk website says they should get 570 free hours per year. Their council's website says they pay childcare providers direct, that "Government funding covers up to 15 hours of high-quality childcare", and that the provider can only charge for additional services such as meals and snacks which parents should be told about "at the beginning of each new term". Clearly that has not happened, as the invoices have only started arriving in the last few months.

I have also looked on the Which website: it says "you should not be asked to pay any additional fees (or top-up fees) in order for you to claim your free 15 hours". Yet the invoices the nursery have sent state the charges are a top-up as the difference between the nursery's fees and the funding they have been given. This seems to contradict the rules of the child care scheme.

We are all at a loss as to where to go, as none of the above sites actually say what to do in the circumstances they are in. The nursery has told them not to send our grand-daughter to them unless they pay these fees backdated to September 2018, which they cannot afford. The nursery also say that if she does not attend, they will charge full fees for a further 8 weeks as notice.
Please can anyone advise as to what the legal position is and/or what we do??
Thanks, Kevin