Ninety years later, what has etched the slaughter of the battle of the Somme into the British memory? A few hideous facts have helped: the 19,240 British dead on the first day; an overall casualty figure of more than one million.

But more than anything it is the work of a film: The Battle of the Somme, photographed on the spot in 1916, planned as British war-boosting propaganda, yet containing in its schizophrenic jumble painful shots of the dead, near dead and bandaged remnants.

At the time almost half the British population saw the Somme film in cinemas, many seeking evidence of loved ones; and there has scarcely been a First World War documentary that hasn’t plundered its images.

On Sunday night this extraordinary film reached two new milestones. One was a certificate marking its entry into Unesco’s registry of documentary artefacts, Memory of the World — the first film, and British item, to make the grade.

The second milestone was the Imperial War Museum’s new presentation of its archive treasure. Many shots had previously looked bleached; digital restoration brought the detail back. And these troops in the mud grinned or stared at us to a new music score by Laura Rossi, brilliantly effective, played with typical dexterity and polish by the Philharmonia Orchestra under the conductor Nic Raine.

Inevitably the music was coloured by hindsight. Though brass rang out in the finale, there was no simple patriotic toot-tooting. We approached the gates of pathos, but avoided mawkish sentiment. Sombre introspection ruled.

Hovering strings surveyed the pre-battle landscape. A wind machine’s eerie gale triggered the first attack. Stretcher cases returned to a plangent oboe (Rossi’s grandfather had been a bearer). The guns’ bombardments prompted synchronised timpani rolls and shivers from the piano’s plucked strings or lower keys. Nothing was overdone here; nothing screamed.

In this music, quite correctly, we heard courage and futility. And plenty of tension: a tremendous help in carrying us through a film flung together in the heat of the moment, never recollected in tranquillity.

Whatever piercing image you might pick — the troops’ cheerful waves, the mascot dog dead with its master, the trench channel’s mud and corpses — Rossi’s score and the IWM’s new print will help them to reverberate even further into the future.