Jim Carroll:-
"Shirley Collins was instrumental in popularising the 'head tone' singing in the revival"

"Interesting!" I thought, "I wonder if there is a satisfactory definition; I'll do an internet search for 'head tone' singing."

I did - and there was one.... on Wikipedia!

I'd be interested to know if Jim can
a) give a satisfactory description of that term
and
b) provide any evidence that Shirley set out to sing using whatever a 'head tone' is rather than singing the way that her aunt Grace, her uncle Fred or her mother or others in her family did.
c) provide any evidence that she was 'instrumental in popularising' such a style.