Could Henry have joined the navy? given he was listed as a ropemaker at one time. There is a Henry Knott joined in 1820 aged 17, on various ships leaving in 1858, which could account for him not being on the census in 1841,1851.
Vicar might have spelt it Nott phonetically on the baptism.
Could have been born South Brent, but not necessarily baptised there.
This one is very puzzling