I don't believe it was John Martyn, he would have gotten some respect. This guy's name was Charlie Starr, he was blind and played the guitar, no other accompaniment. The crowd hated him, absolutely and vehemently. Sublime moment: folks in the crowd giving the finger to a blind musician.
My memory is kind of fuzzy after all these years, but I recall a guy coming onstage by himself with a guitar and then, before even playing a note, walking off. Somebody later told me that it was John Martyn and that he was disgusted with the setup and split.