Hi Ray,
If you see this message - are you still researching John Williams? I'm also descended from the 1870 marriage, but have had the same problems trying to find his birth in the Sarn Bridge area...
The most likely possibility I've found is a John Williams of the right age in the 1851 census, recorded as born in Tallarn Green (I think the closest settlement to Sarn Bridge? I've spent a lot of time trying to work out where the nearest church might be!). He's living with his grandparents John (a basket maker) and Mary Williams, and their unmarried daughter Jane (c.1806), in Penley, Flintshire. John & Mary were in Penley in 1841, this time with their youngest son James (1829). A John Williams & Mary Morgan were married 31 Dec 1804 just over the border in Ellesmere, Shropshire, where the 1851 census records Mary as being born in c.1779 (John was born in Penley - and perhaps christened there in 1779, son of Ann and yet another John).
There's no sign of father John yet... However, Familysearch lists at least six children for John & Mary christened in Penley, including a John born in 1814 - which would be perfect. Findmypast also lists the marriage of John Williams from Penley (father's name John) to Elizabeth Turner at Loppington in Shropshire in 1841 - again a potentially perfect fit - although I haven't ordered the certificate to check John's occupation. (The BMD index records John Williams and 'Elizabeth Tunna' marrying in Wem district in Q3 1841, Vol 18 Page 242.) I've also found a John Williams (aged 38, 'labourer', born Penley) and his wife Elizabeth (age obscured - 51?, born Grinshill in Shropshire) living in Penley in the 1851 census - but there are no children with them.
I don't know if this is the right family or not for our John - but it looked promising. I've also wondered whether John was an illegitimate son of the older John & Mary's unmarried daughter Jane Williams in the 1851 census - so the 1870 name/occupation was just a cover-up - but then 'shepherd' seems unusually specific to me if it was an invention... Hopefully a birth record will turn up somewhere!
Thanks.
Ronan