One factor I forgot to mention, Brian, is the pedlar who amongst his other commodities went from place to place selling the ballads. He would have used the same tune in different places to sell his ballad sheets. Other than that, migration of farm labourers (best example the seasonal Irish labourers), general worker migration was very common. Remember labour was purely a commodity to the entrepreneurs and labourers had to go wherever the work was or starve.

I have no expertise in tune comparison as I can barely read music. I am an ardent campaigner though for the study of tune relationships which is a sadly understudied subject. There are lots of us studying texts but hardly anyone the tunes.