Thanks everyone. Be interested if you manage to get an answer from GWR▸ !
As the National Rail Conditions of Travel are set as an umbrella over all train operators, I believe that the definitive answer needs to come from a central point rather than from within GWR. We may find that there are test cases elsewhere in the
UK▸ which establish the detail one way or another; as GWR account for less than a tenth of the trains operating in the UK, there's at least a 9 out of 10 chance that the answer will be from somewhere else!
Answer I have received from Passenger Focus:
Our understanding of the current NRCoT 14.2 is that a combination of season tickets has to be accepted. Formerly NRCoC▸ 19 stated clearly that a combination involving a season ticket required the other ticket(s) NOT to be a season.
That's the Passenger Focus view ("our") and not the view of just an individual there ("my"). I will follow up in more detail on "frequent posters" ... members posting in this thread who are not yet frequent posters - happy to copy you on my further comments / please just ask.