As BBC 2's epic history series Icons is underway, Tom Service takes a closer look at four icons of the classical music world: Maria Callas, Nigel Kennedy, Jacqueline du Pré, and Luciano Pavarotti. With the help of opera critic Anna Picard Tom asks whether these icons are born or whether they're made, and what factors (beyond great artistry and talent) are at play in the making of an icon.