Many thanks, Vic, for that contribution, and I look forward to reading the Bellamy piece. One interesting snippet I've found on a very brief perusal is WP's account of his grandfather's having learned his songs from broadsides:

My grandfather got the songs from broadsheets, apparently: that's how they were brought round, so they always told me. He could read music, you see; that was unusual...
He learned to play the clarinet, so you see he could take the music off these broadsheets. As he could read music, he got the tune, whereas a lot of these poor old men around here - you never had to go very many mile 'til you hear these tunes altered all out of proportion, because they had'em just by word-of-mouth, and I think that's why his tunes were so good.

It's pretty clear from this that WP had never actually seen a broadside, since the vast majority never included the music. So the reason for the quality of the tunes must lie elsewhere.