For anyone reading who may not understand what Committed Sundays are for traincrew, this explanation might help:
Your contracted hours are 35 per week, Monday to Saturday, with an average day length of either 8.45 (four day week) or 7 (five day week).
You are also rostered to work, as overtime, several Sundays during the year, varying between depots and companies but can be anything up to 50% or so of them.
If you want one of those Sundays off, you cannot use Annual Leave, as they are not part of the working week.
So you have to ask colleagues who are off, whether they could cover your turn instead.
If they can't, you have to work, and bang go any plans you might have had for that weekend.
If however you do go sick, or even "sick", you don't receive sick pay, as it's an overtime day. The flip side of which also means you cannot realistically be disciplined, of course.
So in short, it's a mess, no-one has certainty of staff availability and it makes planning life very difficult too.