Thanks, Tan, for taking the trouble to reply.
Doreen and John Jones from Canada, who have transcribed all the local censuses on Genuki, left a message for someone else researching Williams in Llandderfel in 1851 on the RootsWeb site suggesting that there might be a family connection because of the names. Whether this is actually true, I can't yet prove, as I need to find baptisms from Llandderfel to check Ellis' parentage. I haven't found many other Williams in the immediate area.
Edward and Margaret Williams of Gist Faen are the correct age (60) to be possible parents of Ellis and, as you will see, the family first names, leading to Doreen Jones' suggestion, are the same The other pointer I have is that, in 1851, there is a Jane Hughes aged 7 with them at Gist Faen, and Hughes comes into my family as well, although there is always the proviso that many names are very common.
In 1851, Ellis Williams (b 1812) was living at Maesod, quite close to Gist Faen, with his wife, Catherine (b 1806 - nee Edwards) and their family.
At home were Margaret (b 1836), Elizabeth (b 1840), and Reese Edward (aged 10 days). Two younger children were "farmed out" to relatives - Hugh (b 1842) with his uncle Morgan Edwards at Henblas, and Catherine (my G Grandmother (b 1847) with her grandparent, Rees and Anne Edwards at Llanerch, presumably because of the new baby in the house. Another daughter, Ann (b 1838), was at Garth Goch as a house servant with her aunt Gwen Grestie (nee Edwards) and husband John.
I'm very keen to find definite Williams parents and siblings linked with Ellis (my G G Grandfather), so it would be fantastic if I could get back further. I've come to one of those proverbial brick walls.......
Iolanthe