Hi Tazzie
Well I'll be blowed by this one.
The only evidence of the 1850-51 DOB is from 1881 Census, grave headstone AND details on the son's WWI memorial site. All else points to Samuel Foss as being b.1839-1840 including I now find, the NZ BMD online with his death age 67years. So I think we've found him and family in that 1871 Census you've given.
As to the middle name of BEART I can't see where he's ever used it - none of his childrens birth in NZ BMD online shows that name for the father, just plain Samuel. However, the fact that he gave his son that name says he is connected somewhere along the line to the BEART family - as is the other Samuel Beart Foss - and interestingly, his grandson b.1920 features it as a middle name.
I found an old 2005 post on another site about Samuel Foss and emailed to see if they had established anything - got an answer this morning :-
"christened 10 Jan 1841 Downton Wiltshire UK
Came to NZ 1881-1882
1885 Taratahi Dairy Manager
D 16 Feb 1908 Hukanui Pahiatua NZ
D 15 Feb 1908 age 67 in family bible
1896 Newspaper reports indicate a holiday in the UK after 11 years in the Taratahi job
Son of John Foss and Emma (Beart) Brit Isles Vital Rec idx FHL Film 1279375 dates 1601-1904
M 1870 Aug 30 Samuel Foss 29 Bach farmer and Mary Pool Ker 19 Spin @Christ Church Battersea Surrey entry #189
M by license by WilboreMaeCarthy Curate
Witnesses: James Ker and Georgina Ker"
Did you see that? EMMA BEART was the mother of Samuel Foss?
?
Blimey charlie what a tangled one this is!!!
Best wishes for today!
GAYE