There are two William Jones born 1855 in Llanrug in the 1861 census
RG9; Piece: 4339; Folio: 69; Page: 2
Parents William 40 and Phebe 37 plus 6 children. William is a slate quarryman
RG9; Piece: 4339; Folio: 76; Page: 16
Parents William 40 and Elizabeth 46 plus 5 children, William is a slate quarryman
I can see a problem here as even if you get William and Catherine's marriage certificate you still wont know which one it is as names and occupations, even ages are the same.