Oliver Knussen died unexpectedly four years ago, robbing British music far too soon of one of its most vital and influential figures. Knussen would have been 70 next month, and the London Philharmonic’s celebration of him as both composer and conductor was a timely reminder of how much he is missed. Edward Gardner devoted the first half of his concert to three of Knussen’s own works, and the second to two composers who, in very different ways, had meant so much to him – Britten and Ravel.