As Concerto Budapest performs Ligeti’s Century – Roaming in the Past, Kurtág sits behind its music director, András Keller, shadow-conducting from the score which quivers in his hands, and making the sounds he thinks the instruments should be making. “Yo-reee, not yo-ri”, he says. “Pa-DAL!”, he shouts, his elbow pushing into the arm of his wheelchair. As his opera arrives at the Proms, the avant garde master gives a rare interview for The Guardian.