Will - interesting one. I wouldn't have any trouble saying that Freeborn Man or the Scarecrow or Boots of Spanish Leather is "in the folk idiom". What that actually means & how it could be measured I'm not sure. I think probably the most important characteristic is a kind of declamatory, public style, as if the singer is telling a story or acting out a scene rather than talking to his loved one. There's a kind of singleness of focus, as if the song is there to get a job done & then stop. And, I think, there should be quite a limited vocabulary and a heavy reliance on conventional phrases - which doesn't at all limit what you can get done in the song.

So my definition would exclude Needle of Death; it would exclude a lot of MacColl's work, come to that (and Dylan's, and Lal Waterson's). I mean, I wouldn't say that Sweet Thames Flow Softly, Red Wine and Promises or Mr Tambourine Man is in the folk idiom - they're great songs and I love them dearly, but they're different.