Could it be that the current 'Sunday outside' situation in various TOCs suits everyone except the passengers?
It suits the TOC because it's cheaper to pay overtime to existing staff than employ new staff, which means...
It suits the DfT because the TOC can pay the DfT a bigger premium (or require a smaller subsidy) and...
It suits the staff who don't want to work Sundays because they don't have to, which means...
It suits the staff who do want to work Sundays because there's plently of overtime available, which means...
Everyone's happy
(except the passengers).