Many thanks to all for suggestions. We ended up - well, two of the three of us - following
@30907 's suggestion. The one who didn't for some strange reason stood on the platform at Llanwrtyd as the train - a single 153 - departed. He said he was thinking for some reason that we other two were hiding in the waing shelter or some such. (I think it was a very senior moment).
The 09.09 ex-Swansea was a 2-car 150, and about 1/3 full. Passengers seemed to join or alight at most stations (not Ffairffach, Cyngordy or Sugar Loaf, possibly not Llagennach). I suspect about half the passenges were travelling free on TfW bus passes.
Return from Llanwrtyd to Llandovery was also about 1/3 full, and second 153 from Llandovery to Llanelli similar. Quite a of folks left at Llanelli, so it was down to 1/4 full for the last leg into Swansea.
I must say I was suprised by the speeds achieved, seemed really lively, apart from a couple of PSRs/TSRs (not sure). Quite regular scraping the lineside trees, with some quite heavy bangs. I don't know if this causes any significant damage to the units.
Our lost traveller at Llanwrtyd says he took a 5 min walk to find the Cafe Sospan, which he found perfectly satisfactory cheesy ciabatta. We wandered into Llandovery to look around. Many shops closed on the Monday. Had lunch in the Castle Hotel, my friend had fish n chips and I had a beef curry, the latter very filling. Not exactly cheap by my standards, but I suppose it's rather normal. Nice atmosphere in any case.
The hillsides when the sun was out were a stunning, scintillating green - seemed almost 'fake' caused by tweaking the colour saturation application. There again, I suppose the clue was in the over flowing river, a result of heavy rain recently that saw the lined closed for periods last week.
A great day out. The weather was awful this morning, and I almost thought of calling it off. Glad I didn't. Thanks again to the advisers up thread!