GuestQ: thanks for the input, but after some PMs passing back and forth between Masato and myself I think he has identified which is the earliest of the songsheets (of several marked 1850) in the Levy collection. I assume the version you quote cannot be the earliest as its cover includes the words "A Favourite Ethiopian Song", presumably meaning it has been in use and in print long enough to become a favourite.
It is unfortunate that several of these bear the copyright date of 1850, but this is presumably just quoting when the song was originally submitted to the 'Clerk's Office'. Publication dates may spread over several years but not be quoted on the sheets.

Nigel