There is an article in the EFDSS Jornal which had a comparative set of variant tunes for this song. Now, I am pretty sure that the Watersons version was one of these. Track down the article(sorry, I havent got the reference) and Im sure it will have a footnote on the precise origin
Interesting how many Derby Rams(like this one) had the 12-bar structure and verse pattern that translated well into the Frankie and Johnny, White House Blues, Railroad Bill type of oldtime blues song. And it also morhed into Didn't He Ramble, the classic New Orleans funeral march.(I've done this myself with a brass section...Bellowhead are welcome to try!).