I do have the WR PTM from 1980 (downloaded from the BR Coaching Stock groups.io group) and it shows it as loco-hauled (0715 Oxford-Paignton).
Maybe there was a short-term alteration and it ended up being run with a DMU?
These did seem to happen, remaining somewhat on topic I am absolutely convinced that there was an 1140 Reading-New Street service in 1986, operated with a Tyseley 116 DMU and calling only at the usual IC destinations (Oxford, Banbury, Leamington, Coventry, International). This did not appear in the timetable and doesn't seem to have been in the WTT either, yet I am sure I saw it on both of the two occasions I visited Reading that year, 26 August 1986 and 24 October 1986.
I would guess this was also a short-term alteration, as otherwise there would have been two gaps (at 1140-ish and 1540-ish) in the Reading-Birmingham roughly hourly schedule and this was deemed unacceptable. It would have been relatively inexpensive to operate, presumably, as it could use a unit otherwise idle between the Birmingham peak hours.
Would be great if anyone can shed any light on this service!