I wasn't sure as I have seen GWR recruitment posters in the past stating the work entails "some weekends" - which I assumed included Sundays as well.
If this was the case, how can you have a two tier system whereby some staff don't have to work Sundays, yet all new starters would be forced to. I would have thought the "champions of the worker" - namely the Unions would have had something to say about this inequality.
I think you're inferring rather a lot into this. A recruitment poster stating that the job entails some weekend working does not necessarily mean that it is compulsory* nor that the existing staff are not bound by the same requirements as new entrants. Unless this is known to be a fact it does not prove the existence of a two-tier approach to weekend working. Following the trail of quotes up-thread does not provide any definitive answer to this.
* To be entirely pedantic, Saturdays are also counted as "the weekend" and these
are part of the working week and, therefore, compulsory.