The arresting sight of Sister Wendy Beckett - all teeth and glasses - burst on to our screens in the 1990s. An instant star, she glided around the world in her habit telling us the story of painting. But she revealed nothing of her own, extraordinary story. Was she in fact a real nun? How did she know so much about art? And how could this consecrated virgin and hermit justify appearing on television and keep her rule of silence?