Hello litherland lass,
I wonder how old the house, Plas Du, would have been. Do you have any idea? The reason I ask is that if it is old enough, it would be possible to do a place search on the census for it. That would revela who lived there in years gone by. Then we can see if these people are related to your ancestors.
I suspect the father of Gwen Williams might be Ellis Williams, that the name has been mitranscribed in the register. I note there is a marriage of an Ellis Williams to Anne Humphreys in Llangybi on 20 June 1835. That was also an area where there were slate mines according to Genuki.
http://www.genuki.org.uk/big/wal/CAE/Llangybi/index.htmlThis couple had a son, William, baptised 09/05/1837 at Capel Helyg, Llangybi.
What information do you have on this family from census returns?
It should be possible to track Ellis/Elias Williams and Anne in the census both before and after Gwen was born.
Jo