For those suggesting a change of employment contract to provide a 7 day a week service without any Sunday overtime turns being needed, there are various problems:
1, If no overtime is worked, you either need to employ far more staff (which costs a lot more) or have a drastic cut in the timetabled service. The DfT won't allow the staff costs to rise by that amount and they will not be happy for the amount of train services that would need to be cut.
2, If Sunday becomes part of the working week, but the number of staff does not increase substantially, you will get more train services cancelled throughout the week rather than just at weekends. Because you would then be relying on the same staff to work overtime during the week. And they can refuse to work overtime.
Basically this is a problem caused by people at a high level meddling without either knowing the consequences or not caring about the consequences.
The same problem has occurred before and once the staff find that they prefer not working overtime, you then find that there isn't an easy fix.
The unions will be happy to discuss the problem if the TOC and the DfT are going to negotiate in good faith with the aim of working out a solution that is agreeable to the union members, the TOC and the DfT. But don't hold your breath.