Deep Purple’s biggest song nearly went unheard. “It was a filler track,” says frontman Ian Gillan of the rock classic “Smoke on the Water”. “We were seven minutes short of a record and we only had a few hours left to finish it.” It was December 1971 and Deep Purple were recording in Montreux using The Rolling Stones’ mobile studio. “So we dug out this jam we used for soundcheck. It was called ‘The Dan Dan Song’.”

Up against it, Gillan wrote the real-life story of the night before, when the band watched on from their hotel as a fire broke out at a Frank Zappa concert at the nearby casino, the smoke drifting over Lake Geneva. “We just thought it was nice and part of the album,” drummer Ian Paice says. But an American executive saw the potential, and made a radio edit of the track. “Bob’s your uncle,” Gillan says. “It stuck.”

That’s one way of putting it: “Smoke on the Water” went on to become the most famous guitar riff in rock history. “It’s also the most annoying riff for people who work in music stores,” Paice smiles.

Even all these years later, they still love to play it live. “And we don’t just go through the motions,” Gillan says, admitting they often subtly change its style and tempo. “When I was working with Pavarotti, he said, ‘Ian, I’m so jealous. I’ve heard you sing ‘Smoke on the Water’ six times, and every time, it’s different. If I was to do that in the world of opera, they’d crucify me’.”