"We have a specific and identifiable body of songs, we have an attempt to identify that body of songs socially and culturally and up to relatively recently we have had a consensus on what those songs are - that is our 'definition'."

It depends what you mean by 'relatively recently'. I first became aware of folk music in the late 1960s and even then the term was widely understood to include modern as well as traditional folk. The big stars of the day were the Ian Campbell Folk Group, the Spinners, and the Dubliners and they all included modern songs as well as traditional ones in their repertoire. Of course the REALLY big stars were Dylan, Simon & Garfunkel, Donovan, Joni Mitchell... all widely understood to be 'folk'.

Can you please say when the period of linguistic purity you refer to came to an end? From my own experience I know it must be more than 50 years ago.