They're aren't enough traincrew full stop.
There will be a large exodus of rail staff of all grades after the back pay is paid.
Time plus 20% for station staff or time plus 25% for traincrew will not get people to come to work on a Sunday when there's "easier" overtime in the week even if it's only flat rate as it is in ticket offices.
There are GWR▸ (Great Western Railway) managers systematically working trains on Sundays for years, wouldn't they be better served recruiting and training new staff ?
Your example about two train in three minutes yesterday to Gloucester though is only because of CrossCountry Plymouth to the north terminating short due to engineering works.