It appears that nine railcars were based in South Wales at the time of Nationalisation.
Looking at the Michael Clemens Railways site I found this timetable for 1949-50 which has diesel services shown from Swansea High St to Carmarthen, Pembury, Porthcawl, Port Talbot, Cardiff, Newport, Gloucester and Cheltenham.
http://www.michaelclemensrailways.co.uk/?atk=594
The services are shown at the end of part B and beginning of part c of the pdf files.