It sometimes seems as though there can’t have been a British soldier in the First World War who didn’t stagger into battle with a copy of A Shropshire Lad in one pocket and a notebook of his own poetic musings in another. The conveyor belt of 1914/18 anniversaries has unearthed so many – and among them is Charles Sorley, a 20-year-old soldier-poet killed at the Battle of Loos. His body was never found; only a kitbag full of verse. And it’s become the basis for an oratorio by Sir James MacMillan which has just received its London premiere at the Barbican.