What that actually means & how it could be measured I'm not sure.

Exactly, Pip. I think all of us could say about songs on the margin of the folk idiom, such-and-such sounds OK - and such-and-such doesn't sound OK. Where the line is drawn is probably individual taste.

I should add that, as a musician, I come at traditional and not-so-traditional music from a quite different perspective - which is tunes and not words. I'm primarily interested in playing melodies and I'm a sucker for a good melody. Many of those fall well within the tradition and others do not. I get as much pleasure, for example, from playing Jay Ungar's "Ashokan Farwell" and Tom Anderson's "Da Slockit Light" as I do from playing "Donald Cameron's Polka" and "Beatrice Hill's Three Hand Polka". Words take on less importance for me - which doesn't mean that I don't care for the songs, just that it's the tunes that move me!

So, in many sessions I attend, I find there's less worry over the provenance of a tune and more interest in the musical character and performance of a tune. In the monthly session I run in my local pub, the emphasis - whatever the tune being played - is to encourage and stimulate improvisation around the tune. Which I think is also in a worthy tradition.