Only just saw this thread, though I was at the launch and am now about a third of the way through the book. I wish Brian luck in trying to precis it: there's so much information of all kinds and I'm not yet perceiving a clear overview.

One point that Steve made at the launch is that the essential difference between his book and Bert's is that his is based on firm evidence. The chapters that I have read so far mostly set out the evidence rather than draw conclusions, but maybe those come later.

One point that Steve doesn't make explicitly in what I've read so far, though perhaps later, is that the songs that were being sung at any given time were of various ages but a lot of them fairly recent, at least in their current forms. Of all the songs that are being sung at date X, by a later date Y some will have fallen by the wayside and a some new ones will have entered circulation. Some have lasted for several centuries, but not really very many.