But the reality is that there is , and always has been , some sort of requirements for drivers to work overtime voluntarily. That has always been the case and will continue to be so. The reason it is 2 months is that it coincides with a new rest day agreement which was due to start on 2nd June and end on 27th July.
I am aware of voluntary overtime, but it is different to rest day working (even if some people seem to use terms interchangeably)
My point is voluntary overtime might be short notice offer to help cover couple of hours when someone is sick. It is quite weak to base your staffing strategy on regular cancelling in advance (p-code) which is what northern are doing.
P-code is not supposed to be a substitute for failing to have staff allocated as spare (as part of their normal annual roster) to cover these last minute sickness and unavailability, without it being their days off.
No one should ever plan to be above the average level of voluntary overtime, because guaranteed to be short half the time, should plan to assume voluntary overtime levels will be much lower because being short short of staff should be exception, not a daily norm.
Yes there might be last minute changes, and last minute cancellations, but in a properly managed company you don't contract to operate a timetable that you can't staff routinely without needing people to come in on booked days off.
I agree with your point, always going to be some voluntary overtime, but that is fundamentally different to cannot manage everyday, (or specifically over 250 trains per week) rather than occasionally have problems without some overtime