Erica Wagner is not claiming a history in folk song; she was writing a review of a book by Sheffield-based J P Bean. You can read extracts from the book on Amazon. It is largely based on interviews with people who were around at the time. The quotation about the nail polish is from Shirley Collins, as it happens. The book appears to deal with changes in the folk scene: it comments that the audiences got older and older and that it was difficult for new people to go as it felt cliquey. It looks like a book worth reading, by the way, so thanks for the link.   

I looked up 'flow sweet river'. It isn't a folk song. And it is to be supposed that it was never presented by Maccoll as such.