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