First of all, thank you very much to all of you for taking an interest and being so helpful.
The route by which I came to David Welch and Rachel Merriman was through the original death certificate of 1875 of their grandson (I think), (Joseph) Thomas Davies (aka David), who was the son of Benjamin Davies and Mary Anne Propert. I found the marriage of this couple and traced them on the 1841 and 1851 census records (Benjn David aged 39, and Benjamen Davies aged 46 respectively) with his wife and children, including Thomas aged 11 on the 1841 census. If I have the right Benjamin - and I haven't been able to find another for the right place and time - he is my g-g-g-grandfather on my dad's mother's side (Davies). I then worked back from this to find Benjamin's parents: David Welch and Rachel Merriman.
The records I've used are from the ODM transcriptions on Dusty Docs, which links into the Church of the Latter Day Saints Family Search site. Unfortunately, they often give limited information from the original records they've transcribed.
As for the name - I've found the same children and baptism dates as you for Benjamin (bap.1803, St Issells) and brother David (bap.1809, St Issells), but with the surname David and parents David Welch and Rachel Merriman (I also found one for John David in 1800 in St Issell to this couple). The transcripted records I've found clearly say that the father was David Welch and his son, Benjamin's surname was David. I've also seen quite a number of other records of this time (and later), where the child's surname is the father's first name, which is why I've found this all a bit confusing and not been sure what names I should look for going further back.
One of the biggest problems I've had with all my family research has been having to depend on other people's transcriptions. Having done some checking of transcripts for one genealogy site, I found some pretty awful errors, which was quite disheartening. Anyway, Welch or David, I'm pretty sure I've got the right people.
I shall now have a dig around with the information you've very kindly provided and see where that takes me.
Thank you all so much for your interest and help