Brian, when do you think folk musicians in England started using harmony?

Well, Joseph, that depends on how you define 'folk' and 'musician'. Some village band musicians (fiddles, woodwinds etc) around 1800 were able to play in parts. There's some evidence for part singing of folk songs in the 19th century, possibly deriving from church or glee club singing. The singing tradition in England - which is what I'm talking abut here - was very largely performed solo and unaccompanied right up to the second half of the 20th century. Guitars came in with the second folk revival, when skiffle musicians started turning their attention from American to English songs from the late 1950s - 1960s.