On 20 July 1942, Time magazine led with a story on ‘Fireman Shostakovich’. ‘Amid bombs bursting in Leningrad he heard the chords of victory,’ the caption on the cover said, under a picture based on a Soviet propaganda photo taken on the roof of the Leningrad Conservatoire in September 1941. Shostakovich’s Seventh Symphony, dedicated to the besieged city of Leningrad, had received its American premiere on 19 July 1942, played by the NBC Symphony Orchestra under Arturo Toscanini. On 22 June, the first anniversary of the Nazi invasion of the Soviet Union, it was broadcast live by the BBC: the London Philharmonic Orchestra conducted by Henry Wood raced through the score, finishing four minutes earlier than the scheduled time, to the studio manager’s dismay.

Shostakovich began work on the piece in July 1941 and finished it in December, in the city of Kuibyshev. The symphony was first performed there on 5 March 1942. Meanwhile in Leningrad, the most devastating winter of the siege was drawing to an end, with the daily bread ration going down to 250 grams for workers and 125 grams for white collars and their dependants, and yet there was music in the city. The conductor Karl Eliasberg resurrected the Leningrad Radio Orchestra to perform the Seventh Symphony on 9 August 1942. The story is at the centre of Leningrad and the Orchestra That Defied Hitler, a new documentary directed by Tim Kirby, to be aired on BBC2 tonight.

One of the interviewees recalls her 18th birthday in January 1942, when she put her grandfather’s body on a sledge and took it away. She remembers seeing a Christmas tree with what looked like parcels under it and then realising they were dead children. Her voice sounds incredibly young when she talks about the performance in the Philharmonic Hall, which looked just the same as ever, and the sense of elation everyone felt as they listened to Shostakovich’s music.

The symphony was recently performed by the St Petersburg Symphony Orchestra, conducted by Maxim Shostakovich, the composer’s son. ‘I was scared,’ he says in the film when he remembers attending the Kuibyshev premiere as a three-year-old. He also describes his father’s visit to the ‘confession factory’ on Liteyny Avenue. Shostakovich sat in the waiting-room until someone told him the investigator who had summoned him no longer worked there, having already been shot.

When the score of the symphony was flown to Leningrad in July 1942, Eliasberg thought: ‘We’ll never play this.’ The piece, 78 minutes long, written for an orchestra of more than a 100, is physically demanding, and some of the starving musicians could barely carry their instruments. On learning that the drummer Zhavdet Aydarov had died, Eliasberg went to the makeshift mortuary to pay his respects. Leaning close to the body, the conductor heard the drummer breathing. Aydarov was saved and went on to play on 9 August; for the ‘invasion theme’, the snare drum repeats a two-bar pattern 171 times.

The Leningrad performance was broadcast both to the nation and to the enemy. Years later, visiting the USSR as tourists, a couple of former German officers sought out Eliasberg to shake his hand (he refused) and to tell him it was the music coming out of front-line loudspeakers in 1942 that made them realise they’d lost the war.