Reposting for clarity with italics corrected:

Jim wrote:
As our knowledge of the oral tradition onLy dates as far as the beginning of the twentieth century - anything before that is a guess and way out of the reach of definitive statements

This gap in our knowledge is one that Roud's book sets out to fill, using sources like John Clare - who, in a happy coincidence as far as this thread is concerned, wrote down a full version of The Demon Lover, as sung by his mother in the first half of the 18th century.