Stower - My copy of the Roud index lists those two as the only sources for the song. Neither Palmer's publication of Bob Roberts' version nor Kennedy's publication of Phil Hammond's version add anything to what you have. Kennedy quotes Hammond on the subject of the song - more or less your 3rd sentence above - and also adds that the same story is told by fishermen of Mevagissey in Cornwall. Kennedy doesn't say the Hammond wrote it (or claimed to), and I would have expected him to mention it in his notes if he had.

Mick