Hi Graham,
Checking the marriage and baptisms you mention, the man in question is called Robert Martin.
I think this is the correct 1841 census record for Robert Martin's wife (now Elizabeth Bennett).
1841 census: Portland St, (Walworth), Newington St. Mary, Surrey (HO107/1064, book 6, f 24, p 40)
Elizabeth Martin, 55, N
Eliza Martin, 12, Y
Robert Martin, 7, Y
Elizabeth Bennet, 40, N
Isabella Bennet, 2, Y
Elizabeth Bennet, 1 Month, Y
Edwin Sercombe, 7, Y
I would be willing to bet that the Elizabeth Martin in that household was Robert Martin's mum. It looks like from the 1851 census, Robert Martin (aged 7) was also Elizabeth Bennett's child. So presumably Robert and Elizabeth's son.
Have you been able to find a baptism for Robert, c.1833 in Walworth or nearby?
If that is Robert's son then that narrows down a period for his death from c.1832 to 1838.
Alexander