Now just to clarify -- DOB7 posted:
"The 1841 census shows a candidiate for Sampson aged 51 living at Sancreed, Cornwall HO107 piece 144 folio 14/36 page 8."
and that is the Sampson I was then following around:
"There is a Sampson Harris, 60, mariner, born Creed, Cornwall, in Megavissey, Cornwall, who bears a strong resemblance to the Sampson noted in the 1841 census. With wife Mary."
and that is the wrong Sampson Harris in both years.
At least he's ruled out -- leaving you with no Sampson in a census in either 41 or 51.
I do wish you had said earlier:
"Sampson Harris was born in 1812 in Plymouth, son of John and Elizabeth Harris."
(and also about the remarriage) because that is the information that is needed when searching for someone, and I just assumed you didn't know.
I would imagine Sampson was on vessels a lot of the time (a search in 1851 using "vessels" as civil parish doesn't turn him up either).
These are the 1861 details in Devon St Andrew, which could help with searching:
Sampson Harris, 48, Engineer RN, Devonport
Mary A, 33, Tavistock
Frederick, 9, Sheerness
Elizabeth A, 7, Sheerness
with a visiting RN engineer and his much younger wife and her daughter, and a house servant.
Sampson Jr was away at school? Yes, in Greenwich.
Is there an address on the 1851 birth certificate, btw?