Petros, I am thinking the possible reason for your statement of arrival being after 1856 is that the “family tree” lists the birth of a child to the couple in 1856 in England. This child was Sarah Martha WEBB.
The tree owner may be mistaken about this birth and this is perhaps it.
In 1854
Births Sep 1/4 1854
WEBB Sarah Martha. St Pancras 1b/139
She married and died in Australia.
Sue