The point about Nanci Griffith is not that she's American, it is that she has almost no influence on the UK folk scene. Other non-UK performers(Cara Dillon, James Taylor, Judy Collins) are appreciated and influential musicans here, and if you go back further you could also include Joan Baez and Bob Dylan as being seminal influences on the music scene in Britain. Performers such as Sara Grey make regular tours in the UK and are well known. In recent years, bands from Sweden, singers from Canada and music from France have all made their contribution.

But Nanci Griffith?

Marje