A bit of a rush job I'm afraid. Using the 1911 census the only Martha Jane David found in the 1901 census was under the surname Davies. This is in no way unusual for the times. She was the 5th of 7 children of John a 37 year old collier & Sarah age d 34 Davies of Clifton Villa, Pontyrhyl Road.
The birth of Martha J was registered in the March Quarter (Jan-Feb-Mar)of 1896 in the Bridgend District. Births are indexed according to the date of registration and not the date of birth so Martha could have been born in 1895.
The only marriage found was Martha J David to Llewellyn Roberts Sept Quarter of 1917 in Llanelli District with son Ieuan registered in Sept (Jul-Aug-Sept)Quarter 1919.
Hello Paul, I hope I have found the correct family for you. My maternal grandfather who lived all his 87 years in the Garw Valley, was a cousin to William Hopkin, Fforchwen Farm, as mentioned in your request. William married Elizabeth David in 1910, however some birth references(post 1912) give Elizabeth's maiden name as Davies. My own Davies changed from David to Davies in 1837 and two other separate relatives in the Maesteg area changed to Davies during the 1850-1860s.
Looking at the 1891, 1901 and 1911 census I am inclined to think that Martha Jane (Mrs Roberts) was a younger sister of Elizabeth (Mrs Hopkin). It is widely known that individuals were often employed by family members, but not recorded as such in the census, e.g. sister, nephew, cousin etc. etc.
The potential parents of Martha Jane and Elizabeth were John and Sarah Davies of Pontyrhyl, 1891 and 1901 and of Pontycymmer 1911 census.
The marriage certificate of Martha Jane David/Davies and Llewellyn Robert in Llanelli Reg. Dist. would be the best source of information about the names of the two fathers. Please remember the address on the certificate will be that of bride & groom and neither were necessarily living with parents. The Registrar in Llanelli can help you locate the marriage, cheaper than from London, but ask Llanelli first.