Hi Alan,
Got an 1841 census CD replacement this morning, but cannot find anyone remotely resembling my Griffith Williams (he would be about 27) in Llanllyfni, Llaniestyn or Lamystumdwy. Lots of pages illegible, so... There is a Griffith Williams (27) farmer with a 25-year-old Jane Davies (no relationships in 1841 census) in the same Lamystumdwy household, but... Shall cast the net a little wider. In the meantime have sent off for birth certificate copy for Griffith's second daughter Catherine, born 1841/42. That should provide some clues. Next stop parish registers.