Hi
I'm getting a bit confused here and wonder if you have two different couples?
Presume this is the man you are talking about, in St George Hanover Sq area 1851
James Stacey 30 - dyer [?] b St Pancras
Elizabeth Stacey 39 - b Northampton
Bei Stacey 10 - daughter, b St James
Alfred Stacey 6 - b St James
William Stacey 9 [4?] - b St James
Robert Stacey 2 - b Marylebone
James Stacey 1 mo b St Georges
HO107, 1475, 148, 30
Now on 1861 there's this chap in St George Hanover Sq
James Stacey 40 - bookmaker b Marylebone
Mary Stacey 48 - binder b Tonbridge Kent
James Stacey 13 - b Marylebone
Thomas Stacey 11 - b Marylebone
RG 9, 41, 46, 12
Note the change of occupation and different children. this chap appears to be on 1851 in Marylebone.
There's a death of a James Stacey aged 40 in Jan-March 1861 - ie before the census, so I'm not sure why you know your James marries/is involved with Mary if he dies before they can be enumerated together and you don;t have the marriage, unless you are looking at this Mary