This is from 'An Indiscreet Guide to Soho', by Stanley Jackson:
'How typical of Soho is the story of the organ-grinder who was "playing" that piece from (Mascagni's) "Cavalleria Rusticana" in Dean Street, when he was stopped by an old man in a shiny black overcoat.
"To fast," he said excitedly. "Much too fast!" He seized the handle and began turning it slowly with one hand while he conducted an invisible orchestra with the other. Having finished the piece he dropped sixpence on the organ lid and was about to depart when the organist stopped him.
"How do I know you are right?" he demanded. "I've been playing like that for three solid months. "
The man black drew himself up proudly. "Nobody can tell me how that should be played," he declared. "I am Mascagni."
The following day the piece was played to a slower tempo and, chalked on the side of the barrel organ, was the simple legend, "Pupil of Mascagni."'