Lyricism, the singing line, has always the vital spark in Hans Werner Henze's music, however much it has shifted its direction and changed its emphasis over half a century of composition, and his work list includes a number of scores especially written for great singers of successive generations. Ian Bostridge is the latest to acquire a bespoke Henze cycle; the 50-minute Six Songs from the Arabian, which Bostridge and the pianist Julius Drake introduced to Britain at the Wigmore Hall on Friday.