Thank you for all your very helpful suggestions yesterday .... I am now waiting on the RO to come back to me to confirm some details, and I will apply for the relevant certificates ....
However, I am still having problems find JOHN NASH on the 1851 census. I am guessing he's been badly mistranscribed ..... ggggrrrrr
According to earlier, and later census records, John was born circa 1816 in Neath.
He is in Cyfarthfa on the 1861 census, and I have him up until his death in 1891 (in Merthyr/Pentrebach) .. he just seems to have disappeared on the 1851 census ...

His original occupation was a Mason, so could very well have travelled ... but on later census was shown as a Rail Presser
Any help would be really really appreciated.