In what sense, GuestGerry, do the Americans categorise acoustic musicians Simon & G & those others you name above as "Rock", which surely implies influence of the Rock'n'Roll movement of the mid-50s [Bill Haley & his successors], with its ostinato amplified beat, long jazz-influenced, in sense of being semi-impro'd, instrumental breaks &c?

I think this matters, because over-extended use of any term [what I have heard semanticists categorise as 'over-defining'] robs it of its meaning? So that if you call S&G 'rock' musicians, what term are you then to use of lineups like Fairport et al who set out deliberately to introduce Rock'n'Roll techniques into the performance of folk music?