Thanks for your comments!
Just to make my puzzlement over all this a bit clearer, let me put some figures on the particular journey involved.
Occasionally I travel from Nailsea and Backwell to London Paddington, at, say, two or three weeks' notice. I check the website and find that, for example, the cheapest ticket for the journey from Nailsea at 0646 and returning from Paddington at 1700 is ^118.
However, if I buy a ticket from the website for a journey from Bristol Temple Meads to Paddington and back, it's only ^106. On the day of travel, I then buy my usual standard day return from Nailsea to Temple Meads at ^3.80. Total cost, by splitting the tickets, ^109.80 - a saving of over ^8!
Now, the bizarre thing to me is that this is for exactly the same train - an
HST▸ that goes right through, leaving from Nailsea at 0646, and Temple Meads at 0700 - both well before 0900, when cheaper off-peak tickets could come into play.
While I'm obviously quite happy to save myself ^8 in this way, my only concern is whether I should hop off the train at Temple Meads and then straight back on again, just to 'split' my journey in the same way! Vacman, any guidance on this point?