Hi Heywood,
Thanks for the information - the mystery deepens!
At first glance it seems to be the correct family - I'll explain, on Thomas' marriage certificate 2 of the witnesses are William Jones and Elizabeth Pugh, who I suspected was a sister, and yes, she is.
Or at any rate an Elizabeth Jones (born 1847ish) married David Pugh and they appear on the 1881 census in Gelligaer (don't have details in front of me to confirm the exact reference sorry) with Elizabeth Jones, widow and Mother-in-law of David Pugh, who is born ca 1810 in North Wales. These all tie in with the references you have given.
On Thomas' marriage certificate of 14.7.1873 he is aged 22, which doesn't match the baptism record either, and might account for Thomas' abscence on the 1851 census (ie yet to be born)?
All I have is a photocopy of the Church list for the year 1850, so now I'm wondering whether I have the correct record (must be plenty of Thomases born to a David and Elizabeth Jones in Rhymney at that time).
I am now going to check the 1871 census.
Thanks for your help
Helen
