This morning I spent an hour in the library. I found the only feasible death of Thomas Williams in Llanelli Aug-Oct 1918, aged 87, birth estimate 1831. That would make him 2 years younger than Margaret which is consistent with census records. I first assumed, being estranged from his wife and children for many years, he would be buried in Llanelli and not a bought plot.
As for Margaret Williams, I cannot find anything other than the death in 1908 aged 72 years. I will write to the GRO asking them to clarify that from the original document, as I feel it should be aged 77 years.
The archive service have restarted providing some support in the library on a Thursday morning. Once I have more information I will book an appointment.
Later, I called at the chapel graveyard. I looked for the missing fragment which is quite a large piece of slate, but nothing. It was a long shot. Looking again at the main fallen piece and the stump remaining; something did not fit if they were placed together. Margaret had been added, but to the side and beneath there is the damage. Along from it is the date [damage]9th 1918, but they do not align; the date is considerably lower. I had been connecting Margaret Williams with the date, but now feel the grave holds three people. Could it be Thomas Williams? As the plot had already been bought, it would cost very little to bury him there. With burial records lost I will probably never know. I have the quarter, so the Carmarthen Journal may contain something. Although he was a drinker and fighter, always in court, or prison and then the asylum, he was quite a character, so his death may be worth writing/reading about.
After 1889, the gravestone would be erected for the 22 year old, but in 1908 and 1918, it would need to be removed for new information added. At some point the stone has split across and repaired with an iron pin, which is now where it has broken. The pin, when it corroded would have expanded, to cause such a fracture. In answering one question, more questions arrive.