Coincidentally I received my 2nd reply from NLW today., from Bishops' transcripts in Aberystwyth. I now have it confirmed that Lewis Prosser(bapt. 14 June 1774/5,( I assume that means 1775?) was the son of Thomas Prosser,(1716 - 1777) of Ty Du, Merthyr Cynog. Also that there was a Benjamin baptised to Thomas in 1760, from censuses and Llandrindod records I know that he too lived at Ty Du, until his death in 1852 and that he was married to Elizabeth Jones in 1790, being a widower then. Probably his previous wife was Margaret Williams, married 1786.
Now I would like to see how much further back we can go with Thomas, so if you are in Aberystwyth any time, could you look up his baptism around 1716 and see who his father was.
Eternally grateful!!
Helen