In case it's of interest, this could be your Sampson's baptism -- the 1841 census does indicate that he was born in Cornwall c1790:
http://www.cornwall-opc-database.org/search-database/more-info/?t=baptisms&id=2099195Son of Peter Bown Harris and Ann, baptised 31 Dec 1790 at Budock. Peter Bown Harris was baptised 1766 in Mawnan, father Sampson, mother Mary Bown, is how it looks -- you can search the databases there and see the various records.
But your question was about 1851.

Here is Sarah Hannah Harris, I think:
Sarah H Harris, 14 (c1836-37), born Devonport, scholar at boarding school, Woolwich, Woolwich Arsenal.
The head of household is a widow and her daughter is the schoolmistress. It's quite a nice neighbourhood; the previous household has servants who include a governess and a lady's maid. The next household is headed by a private tutor and inhabited by gentlemen preparing for the army.
This doesn't help find the rest of the family, unfortunately. But I'm thinking ... it's kind of looking like it may have broken up.
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.
There aren't a lot of Sampson Harris-s in the BMD records. Megavissey is in St Austell reg dist. A Sampson Harris died there in 1858, before ages were recorded. After ages were recorded, there is no death of an SH of the right age. Mind, a Sampson Harris did die in Bermondsey in 1853, and I don't see that person in 1851.
Do you know any more about the family? Births/baptisms of children, death dates ... ?