You have been looking for the 1851 census for your Alfred Seymour.
Remember his mother Mary re-married in 1843 to Thomas Wood
In
1851 census all the family (including Seymour children) are using the surname WOOD
https://www.familysearch.org/ark:/61903/1:1:SG18-2MK?lang=enOccupations for Alfred and for Rebecca are not recorded, but I would assume they were both employed.
It looks as if Mary had 2 sons born to her second husband.
BIRTHWOOD, WILLIAM Mother - STONE
GRO Reference: 1845 S Quarter in SAINT GEORGE (IN THE EAST) Volume 02 Page 95
I can't see a registration for the son named Thomas born abt 1842 - maybe not registered.