Getting confused (and sidetracked) over Griffith Williams. His marriage (in 1845 to Ann Thomas) certificate says he was a farmer living at Dolwenith, Llanllyfni, and that his father was William Williams, also a farmer.
But in the 1861 census (thanks Alan) he is a master joiner, with his own business!
I wonder if he 'built' 12 Uxbridge Square for the family prior to his death in 1867.
Any chance of finding them in 1851?
Thanks everyone (massively) in helping me out on this.