Have made some progress!
I know David Thomas (Snr) had a son David Thomas born around 1859 Abergwilly. David (Snr) first wife was Mary Richards. There was only one birth entry for the son that matched this criteria, so I ordered and have now had this back. David Thomas born 21 May 1859 Abergwilly, father David Thomas, Farmer. The birth address is Troedyrhiw which at first did not make sense but then when I searched for Troedyrhiw in census records it all started to fit together. Troedyrhiw, Abergwilly is actually the Thomas’ family home. In 1851 we have at Troedyrhiw
John Thomas, Head, 44, Newchurch
Rachel Thomas, Wife, 40, Abergwilly,
Elizabeth Thomas, Dau, 14, Newchurch
Thomas Thomas, Son, 2, Abergwilly
David Thomas, son, 18, Abergwilly
John and Rachel are still at Troedyrhiw in 1861 (I can’t find them after that)
In 1841 the family are at Glyn Velin, Newchurch.
John Thomas (of Newchurch) marries Rachel Thomas in Llanpumpsaint on 09 Feb 1832.
Going back to the original query about David Thomas born 1859 who married Margaret Aubrey. I can’t find him in census records prior to 1891. Conversely I can’t find David Thomas above in census records after 1881. There is a suggestion from descendants that the 2 David’s are the same person. The confusion about tracing them being that David that married Margaret gives his birth places as Llanelly, Llanarthney and then Pontyberem. The other David giving his birthplace as Abergwilly. It was proposed that Abergwilly is the correct birth place and up until 1881 would have been provided to the enumerator by his father. David’s father died in 1888 and therefore in 1891 onwards he would have had to provide his own details. He may have been unsure, therefore in 1891 defaults to Llanelly (the birth place of his step siblings, in 1901 to Llanarthney, the birth place of his stepmother and in 1991 reverts back to Pontyberem/Llanelly.
There is some merit in this, but thee is not an exact fit, as David who married Margaret gives his birth date of 01 May 1859, whereas the birth certificate of the other David gives 21 May 1859. Close, but is it close enough?