Also in the 1841 Census are William's parents and siblings, living in Deptford.
Grove Lane, ecc.St.Paul, Deptford, Greenwich, Kent
Alexander Johnston, 55, Labourer, b.Scotland
Ann Johnston, 55, b.Kent
Elizabeth, 20, b.Kent
Georgiana, 15, b.Kent
Susannah, 13, b.Kent
Amelia, 11, b.Kent.
That makes Alexander b. around 1786, and it looks as if William was the only son.
Finally discovered some Scottish blood in the family history!