I may have my dates wrong then!
That working applied in one form or another from the late 60s - apart from anything else, it gave the WR time to maintain their stock (and 8-9 cars on the Basingstoke/Salisbury would have been overkill).
Makes sense, there being no need for the Barnstaple on a Sunday, no doubt the set got used for something else
From memory, the hauled sets were a Channel Islands set (on the 1638 etc) and the Newspapers set (0140 WAT-YVL etc). However, the actual workings changed over the period.
Fairly certain there have been previous threads on this period.
(Note - the CI service needed 2 sets SX, with a long layover from 1000-0930 until someone at Wimbledon spotted an opportunity/need.)