Thanks hanes teulu. I've spent two days now trying to find various birth and census records for Benjamin David, Mary Anne Propert, their son, Thomas, and his wife Ann Griffiths and their children. I can find plenty of surname David births around the late 1700/early 1800s in Maenclochog, but the only thing for Benjamin David I have found so far is the baptism to parents David Welch and Rachel Merryman in St Issell's, which is clearly nowhere near the right place of birth for Benjamin shown as Maenclochog on his census records! It is beyond frustrating.
His son, Thomas (my g-g-grandfather), disappears off the census records between the one you found in 1841 and the death record I have for him in 1875. Between 1852 and 1861, Thomas’ parents and siblings moved to Cardiff, but Thomas remained in the Newport, Pembs area and married Ann Griffiths in 1852. It is possible that Thomas was on military service from 1853 to 1857: Thos Davies, 1853-1857, UK, b.1832, Pembs (Dusty Dpcs 1562744, 004650254) – this would also tie in with the births of his first two children in 1852 and 1857. His sea record shows he was sailing from 1858-9 on the Elizabeth; 1863 on the Mary Jane; 1870-75 on the Sylph (Smyrna); and 1875 on the Faithful (Lisbon) when he died of fever in Rouen. This means he should be on the census records for 1851 and 1861, but I have failed to find him so far. I have his wife and children living next door to and partly with Ann's father, John Griffiths, in Temple, Newport, Pembs for 1871, but nothing for them for the preceding years (I'm ok from 1871 forwards). It's as if there's a big hole in the records.
I can't see any point in sending for Thomas' birth record, as I already know who his parents are and that his place of birth was Dinas from his death certificate, and there doesn't appear to be a record for his father's birth to send off for! I think I've hit a wall with this search, to be honest.
Anyway, thanks so much for your help.