I don't believe WMT drivers have Sunday working in their contract. From the looks of it WMT have rostered all their drivers within contract Monday - Friday this week to try and run a decent service (which for the most part they have from my travels).
Correct, NO WMT/LNR Drivers are contracted to work Sundays. Rostering is done over the other 6 days with 4 of them at work and 2 of them off. Long weekend (F-S, M-T) restdays appear every three or four weeks. A simple roster would show a Driver working Mon-Thursday then having Friday, Saturday, Monday and Tuesday off, then working Weds-Sat the following week, then M/T, F/S the third week with Weds/Thursday off, then back round to Mon-Thursday again.
Roughly one week in four Drivers would have a "rostered Sunday"..... and most would work these although all have the option to go "not available" and all have the option to work every Sunday going if they wanted to by volunteering for the non-booked Sundays.
Although rostered they are non-committed and treated as overtime (attracting an enhanced rate of pay), and within ASOS (action short of a strike where non-committed overtime is outlawed) that explains why NO drivers were in work today.
Pretty much every other grade has worked towards committed Sundays, many senior conductors now have a commitment to work their Sundays as a result of contract changes brought about through a past pay deal, Drivers t's and c's have not been touched at all really over the years, even to the point where the ex Central and ex Silverlink conditions still have not been harmonised after the best part of 20 years.