You can thank ASLEF for the 256 farce. The company have just worked around it. You should hear how often we curse and say "if only Plymouth could take it to xyz..."
ASLEF wants Sundays in, there's going to be better adherence to the Fatigue Risk Index, and the company is front-loading the training so it's not miles behind the establishments come May2021's link change to reflect things. I suspect your depot has a lot of legacy "I've never worked a Sunday" types, so I can see why anything with Sundays that need to be worked would be unfavourable.
The company doesn't want to be fighting for Sunday coverage for much longer. With GWR grade now having committed Sundays (as always was on West, but spreading to LTV, and now HSS through GWR drivers joining at HSS depots) the next logical step is Sundays in. Having it linked and covered properly helps with managing the fatigue index - something overtime and RDW will impact, so I can envisage that being reduced (which is something ASLEF have always wanted anyway)