Meanwhile, back at Folk Song In England by Steve Roud which is the subject of this thread, though it has not been referred to in the last 45 posts. Here is another on-line review on the book. It is another that is factual rather than analytical, but as has been pointed out, there are few with the background, experience or skill to give a detailed in-depth assessment of the book's implications.
Read it at - http://www.folkradio.co.uk/2017/08/folk-song-in-england-by-steve-roud/