Ah, yes; I overlooked that thread yesterday.  I actually thought it was a reference to this current one; now that I look again, I see that it was a year ago.  That explains why I was getting one of those déjà vu feelings, having posted to that thread also.  Another good argument for keeping discussions of a particular song or group of songs to one thread where possible, perhaps.  Sorry for any confusion caused.  The older discussion is here:  Lyr Add: Lord Beichan and Susie Pye.

It would probably have been out of character for MacColl not to have changed the tune.  Unfortunately, some of those changes remove specifics which would make identification of his exact source easier for the untrained musician (me).  I want to know the answer, now, and will look further.