Thank you for all the help.
I think I have found Henry Johnson in the 1851 census Westminster St Margaret, Middlesex. (Piece 1480, Folio 565, p45).
As he is not living with his parents I need to find a birth certificate for him for his birth in approx., 1838, Hayes Middlesex. I always find trying to trace some one with such a popular name rather difficult.

I feel sure that the 1881 census information is correct and the enumerator must have put down the wrong surname. Hargreaves is not in any way similar to Johnson so thank you for finding this. All help much appreciated. Thank you.