As his play The Local Stigmatic is revived 50 years after its debut, the reclusive playwright looks back at the London underworld, the rise of celebrity culture and the debt he owes to Harold PinterIt all came about after I met Harold Pinter in the barbers. I was young and had just written a book about the speakers at Hyde Park Corner, which Pinter reviewed in the Observer, writing: "These are the only people I'd eve…