No problem whatsoever doing that. Provided your ticket allows break of journey, and you resume later further up the line on the same route.
Thanks - that worked (or I think it did - minimal ticket checks!). A to B and C to D on the return half of my off peak return from D to A, with B to C on a very busy train via single tickets by a split ticket combination that was much cheaper than upgrading my ticket as a whole to peak. Well - that was a new one even for me. Thanks for the confirmation.
As an aside, I read
Not valid on trains timed to depart Reading before 10:30 and after 15:30 until 19:01
as disallowing travel on the 18:45 and 19:16 from Swindon to Chippenham as they are trains that had departed from Reading in the blackout period, but allowing the 18:52 as that hasn't come from Reading.