Looking for something else, I happened on this page, which juxtaposes Bert Lloyd's "Skewball" with a couple of the broadside versions it derived from. The differences are fairly large - it'd almost be quicker to list the similarities.

I honestly don't know what to think about this. Bert Lloyd's version (as sung later by Steeleye Span and Martin Carthy) is much more the finished article than the broadsides, and almost certainly sings a lot better - but I can't help feeling it's a different song.