A few years earlier, 1986, there was certainly a Reading-Birmingham (and corresponding incoming Birmingham-Reading) which was diagrammed for a Tyseley 116.
Looks like it was a last-minute addition, as it wasn't shown in the published timetable, but appeared to run regularly (on the three occasions in summer-autumn 1986 that I spent some time at Reading). It filled in a gap at around 1140 in the hourly-ish Reading-Birmingham schedule; IIRC it was the only hour during the day when there was no loco-hauled XC service so presumably a last-minute decision to use a DMU to fill in the gap, working to Reading immediately after the Birmingham morning peak.
Another example of those quirky, yet efficient (the unit would presumably otherwise be sitting around Tyseley all day) features of the BR era all sadly gone now.
So maybe this is something similar? Wasn't that familiar with the 1991 timetable, I have a bit of a blind-spot for XC schedules in the late 80s-early 90s period.