Once Igor Levit starts playing Erik Satie at 10 a.m. on 24 April at the Queen Elizabeth Hall, he can expect to be there for a long time. Satie’s Vexations is a piece that looks innocent enough, like butter wouldn’t melt in its composer’s ears. A doleful 18-note theme in the bass is filled in with stately, chorale-like notes in the right hand; the theme repeats, followed by the same chorale except turned upside-down.