I feel the cost of bringing Sundays inside is being significantly underestimated in parts of this debate. The unions are actively for binging Sundays inside, and many staff are not against it as it will mean you could apply for leave instead of being forced to work overtime on commited days, along with making Sunday pay pensionable.
However, the uplift in operational staff required would be at least 15%, likely closer to 17% as formalising Sundays will introduce spare Sunday turns and have to account for leave and sickness too. Not only will there be more staff costs, (plus more training costs, more equipment, more managers etc) but all staff will require a reasonable uplift in salary / compensation to agree to the significant worsening of their terms and conditions.
For example, I am committed to working 2 in 3 Saturdays and 1 in 6 Sundays. I can pick up additional Sundays if they suit, paid at an enhanced rate and can usually find cover for my commited Sundays if I would like any of those off. This means that staff who enjoy overtime have regular access to it, and those who enjoy Sundays at home rarely have to work one. To go from this to forcing all staff to work 2 in 3 of all weekend days is a big quality of life hit. I understand (and personally agree) with the need for a full Sunday service, but I couldn't vote to worsen my own work-life balance to such an extent without a significant uplift somewhere else in my T's & C's.
This likely puts the overall staffing cost increase somewhere north of 20% if it is to be implemented without large industrial relations issues before you come onto the additional knock on costs, such as removing track access time for engineering works (less time = larger cost for the same quantity of maintenance and renewal).
All of these additional costs would be against a backdrop of other cost related issues currently facing the industry. At my TOC there have been no pay rises for approaching 3 years, which is starting to cause discontent as I am sure everyone is well aware of what has happened to the £'s purchasing power over that time. Staff / unions will be asking for a significant pay deal to address this, but I am almost certain one will not be forthcoming. Combine this deficit with the uplift also required for Sundays and the difference between the current budget and pay required is (very broad estimate) somewhere around 25% - 30% apart for operational staff. The reduction in Sunday overtime will cover only a small fraction of this.
TOC's are also not immune to the wider economy, and the same inflation that will lead to pay talk issues is also increasing all other operating costs (Fuel etc) and the DfT are in no mood for meeting this challenge at present. TOC budgets are already squeezed so far as to make day to day operations challenging and should any additional money be allocated, it will likely be needed in other areas long before a nationwide Sunday resolution is addressed. I don't particularly like it, but committed Sundays seem the most logical way forward at present as I just don't see where all of the additional funding comes from.