Regarding the Swindon 120s being more gloomy inside than the Gloucester RCW built to the same drawings, I had that same impression if you sat in the front (as I always hoped to). The Swindon front end, just two windscreens and a lot of intermediate metal, seemed to let much less sunlight through than the fully glazed Derby front end on the Gloucesters, which made them, and the saloon behind, notably brighter. Someone else can work out the extra glazed area. Am I correct that the Derby front was actually built on the jigs there and sent down to Gloucester, same approach as later done with Pressed Steel and Birmingham RCW units for the WR.
However the 1969 WTTs which I have just unearthed confirm it - there were in fact two summer Saturday Minehead trains:
0624 Oxford-Reading-Minehead loco hauled, maximum load 9 and slowest timings - which fits with a Hymek - returning to Paddington;
0850 Paddington-Minehead and return, booked DMU - which would have been the Intercity set(s) as otherwise there were only suburban units back then.
I somewhat remember those Oxfords, but did they run via Reading rather than Didcot West curve and Swindon, then either via Trowbridge or Bristol TM ?
The Cardiff-Plymouth service went over to the 123s when they were first introduced, but a Hymek+8 seemed to deputise sometimes, and it wasn't long before the reversion was permanent. The 123s always seemed to growl away noisily but slowly west from Taunton in the late morning, I wonder what speed they went over the top of Dainton at. What was shown in the WTT was a good guide but not always gospel.
Regarding "slowest timings ... fits with a Hymek", that doesn't readily follow. When they were new, D7024-39 went to South Wales, where they fully took over from steam the Swansea/Cardiff to Paddington expresses in early 1962. Even got a mention by John Adams and Pat Whitehouse in Railway Roundabout on the TV. These were always the heaviest expresses on the whole of the WR, commonly 13 coaches, long a real task for the Landore Castles, and quite why they got secondary diesels (until bumped by new Westerns about 18 months later) not apparent. But goodness, the Hymek performance was up to at least Warship standard, if not beyond. Visiting family at the time near Patchway station I used to go down there, where from the Up platform you get a very good view of the crest of the long ascent from the Severn Tunnel. You heard them coming first, that distinctive full power Hymek Yang-Yang-Yang-Yang staccato sound, quite different from the complex burbling of Warships and Westerns (I could also hear Hymeks accelerating a away westwards from Taunton in the middle of the night from our house, a good half mile from the line). Then steadily first the horns on the roof, then the windscreens, and finally the whole loco front appeared as they breasted the summit onto the flat. They seemed to have readily managed 40-50mph up the long drag. If it had been an electric transmission loco the driver would be watching carefully his ammeter red lines. On a hydraulic it was just push the throttle right round and sit back.