I've just remembered that something that I wrote in 1999 on the Musical Traditions website is relevant to this thread. It concerns another of my great heroes, the Scots traveller singer, Davy Stewart. I was reviewing the re-release of Davy's Topic vinyl album on CD on the Greentrax label and I said:-

For those of us who wanted to believe passionately in an oral tradition, here was the best living example. Davie was fully of the tradition and yet he created within it and that's how the new versions developed. I remember once at Kinross hearing him delivering another of his truly passionate pieces, Bogie's Bonny Belle. His knowledgeable audience was hearing that Belle and her tinker partner were selling "pots and pans and paraffin lamps" so they knew they were on the last verse, but Davie was enjoying himself too much to stop, so we had three more verses speculating about what else they might have been selling; pegs, paper flowers, and expanding on the tinker's ware to augment the tilly pans and ladles. The electric atmosphere, the look on those listening faces is something that I'll never forget. I feel sure that there's another example on this album. He is singing The Jolly Beggar in Hamish's kitchen.   Now, I'll wager that some folklorist has told Davie that there is speculation that the central character in this ballad is King James IV, 'the Gaberlunzie King'. Is that the case? Let's put that in. (sound clip) I may be wrong, but I don't think any version actually puts the king's name in. Just to top this off, this version ends with a final verse to suggest that it might be a good point to break off the recording session and have a cup of tea!

It wasn't just that extra verses appeared within songs, the position of the words against the melody changed with every performance and if extra words or phrases appear, then the melody will just have to do what it can to fit in. His version of The Overgate was similar in structure to Belle Stewart's, but Belle's usually came out about the same. With Davie you knew it was going to be different each time. (sound clip)

If you would read the full review and listen to the sound clips you will to click on http://www.mustrad.org.uk/reviews/d_stewar.htm