Hi Rosie99
I think I am going slightly nutty. I know my research up to Thomas Davies, the master mariner, is correct. His death certificate clearly states that he was born in DINAS, son of Benjamin Davies and Mary Anne (or Marieanne - it's in French) Proppert; husband to Ann Griffiths - married at Newport about 1852 (the actual date was 27.1.1852). So that's quite a lot of very specific information. I also know that Thomas and Ann/e Griffiths' children were born in Newport, Pembrokeshire. I've also found a record for the marriage of Mary Ann Propert to Benjamin David on 28.1.1825 in Dinas, Pembrokeshire. Beyond that, it all seems to come unstuck.
I have a working table for the family, and on that I have two different possible birth/baptisms for Benjamin. One is b.ca.1802 in Dinas, Kemess (or Cemais to be more precise), Pembrokeshire . The other is bap.26.6.1803 in Saint Issell's, Pembrokeshire. So, the first is on the northern coast, the second is on the southern coast. It's always feasible for someone to be born in one place, then baptised in another (sometimes several years later in the case of some of my ancestors!) - where the family of one of the parents originated and the grandparents live, for example - which made the link to David Welch and Rachel Merriman in St Issell's as Benjamin's parents.
However, when I saw the birth place of Maenclochog for Benjamin on the 1851 census records you showed me the other day, it appears that neither of the birth/baptism records I've found for him are correct and I need to look for completely different parents for Benjamin. I haven't been able to turn anything up yet. I haven't found any other census entries for a family with parents Benjamin and Mary and/or Ann with a son Thomas the right age to be my G-G-Grandfather, so I'm pretty sure this is the right census family (famous last words!). I think the search route I need to follow now is Benjamin David b.Maenclochog.
Many thanks to everyone for all your help