'and in Beverly Minster it seems that the fiddle & guitar was, in fact, one and the same instrument played by one and the same player.'
My theory, for what its worth is that the Beverley sculptures were done by the same sculptor as did the Gothic Hall figures in Bruges. The faces and style are virtually identical. That being the case I guess you should go to Bruges to check out the Musical Instrument museum. If this fails to enlighten there's always the 400 + Belgian beers to sample before the ferry comes back.