When the pianist Paul Wittgenstein lost his right hand in the First World War, he asked Ravel to write him something he could play with his left hand alone. The result is a masterpiece that glitters like a rare diamond. Played tonight by one of its most brilliant living champions, Nicholas McCarthy, it is the smouldering heart of a concert that begins in the English countryside as depicted by Ralph Vaughan Williams, and ends with Tchaikovsky at his most impassioned.