The song, titled "Roman Wall Blues," is a setting of a W. H. Auden lyric about an unhappy guard during the Roman occupation of Britain. It was part of a 1937 radio documentary, broadcast from Newcastle, for which the 24-year-old Britten wrote the music.
According to the Guardian, filmmaker John Mapplebeck looked for materials from the broadcast for years, but had largely given up the search recently. Then he mentioned the broadcast to Philip Pendrel-Smith, a 99-year-old retired banker whom Mapplebeck drives to church once a week.
Pendrel-Smith, it turned out, had appeared in the 1937 broadcast as an actor; what's more, he had picked up a copy of the music for "Roman Wall Blues" at the end of the performance and was storing it in his home.
The score is now in the Britten-Pears library; the song will be performed later this year at the Aldeburgh Festival, which Britten founded.