I've been reading up on early 70s 'Trad influenced singer songwriter Folkie' Allan Taylor.
He's still working & releasing CDs.

If any bloke might know a bit about "What makes a new song a folk song?"

he might just be the fella... He's only gone and got himself a PHD...

http://www.allantaylor.com/bio/index.html

"It is a unique work in that it is the first time that a full-time professional musician/songwriter has done such a study, using empirical knowledge as a basis for an academic thesis. The three foci of the thesis are:

the creative process
the power of song
the aesthetics of song.
The Ph.D. is called "Song, Song writing and the Songwriter" "