Constructed as a series of variations, Benjamin Britten's 1954 chamber opera The Turn of the Screw is an enigma. The narrator of the Prologue is conspicuously detached from the story, his witnesses unreliable: a halfwit house-keeper, a hysterical governess, two traumatised children and two ghostly predators whose crimes are couched in innuendo. Time blurs in the opera, the decorative trappings of a Victorian, Edwardian or mid 20th-century country house fading from view. Most unnervingly, there is a sense that the turning does not end with the performance, that the circling 12-note theme continues, unheard.