I asked the original question to the
FGW▸ team via the web site ... answer:
I note that you wish t travel between Melksham and London Paddington in September and also hold a Senior Railcard. I also note that you want a reasonable ticket for your journey.
I can suggest you to purchase advanced tickets using the Railcard. AP tickets are offered at a massively discounted price compared to the cost of more flexible ticket options and because of this they're extremely popular and can sell out very quickly. They can be purchased from a range of retail outlets including telesales, online, stations and ticket vending machines and this means that we can never accurately advise a customer of how many are left. Therefore I request you to contact our Telesales Department on 03457 000 125. We also offer ticket sales online via our website
www.firstgreatwestern.co.uk, where you can create an account by following the 'Buy Tickets' link on our home page.
Because this didn't answer my question (at least to my understanding it didn't), and indeed suggested an alternative which wouldn't have the flexibility I wanted and actually cost more for a sample date later this week, I called to ask. And it seems that it is the journey and not the ticket it applies to, from having spoken with an advisor who's name I know, and with a case reference, I now feel I can join the train and buy my discounted journey tickets with a split. But, yes, I will take a note of the correspondence and printout of the railcard website in case it's queried.