I see no arguments in this thread to counter the view that "like the songs of Dibdin and all the rest, the song was quite likely the work of an armchair enthusiast whose closest link with Spain was a pint of sherry in a tavern in Deptford.

I think somewhere above some said or quoted the following - The oldest mention of the present song does not, however, appear until the 1796 logbook of HMS Nellie

I would have thought that if the song had been written by some "armchair enthusiast" there would have been records from the time whoever it was printed it and had it performed (Like all of the compositions of the likes of Charles Dibdin).