The orchestra was the Royal Philharmonic and the conductor for the night was the British composer Sir ­Malcolm Arnold. The event caused a furore and sent shock waves round the musical world. Nothing exactly like it had been done before.

The event was the first ­performance of a new ­concerto – but it was a concerto with a ­difference. It was entitled ­Concerto for Group and ­Orchestra, and the group in question was Deep Purple, a

rock band now of legendary status. Its composer was Jon Lord, the group’s founder and keyboard player.

The concerto has since achieved cult status and ­continues to be performed all over the world. Lord, now in his late sixties, left Deep Purple in 2002 but continues to be actively involved with performances of the concerto: by December he’ll have done it 22 times this year, and more than 30 performances around the globe are already arranged for next year.

He has expanded his repertoire of orchestral compositions, too. He has written a glittering piano concerto, recorded by the South American pianist ­Nelson Goerner, and a very clever and warmly evocative Durham ­Concerto, a set of scenes

inspired by the geography and character of the English city and its great cathedral.

Lord arrives in Scotland this week, with conductor Paul Mann, to put the finishing ­touches to preparations for the belated Scottish premiere of the Concerto for Group and Orchestra, which will be staged next Monday in the Usher Hall in Edinburgh. It marks not just the 40th anniversary of the piece but the culmination of a huge education project initiated by Stevenson College.

The symphony orchestra for the concert will be provided by the Royal Scottish Academy of Music and Drama, augmented by players from the Edinburgh Schools Symphony Orchestra and the Edinburgh Music School at Broughton.

The rock group is from ­Stevenson College, but it too will be augmented for the night. Lord, who has been visiting the college throughout the year coaching the band, was invited to be at the heart of the concert, on stage in the Usher Hall, presiding over the performance from his trademark Hammond organ.

“Working with young people is like going back to the well,” Lord reflects after a rehearsal session with the rock group. “It’s a glory to work with these young people. You’re seeing them at the beginning of their journey. It’s such a huge pleasure to lead them gently towards what they’re actually getting themselves into. I don’t think they realise the size of the task.”

But did the young Jon Lord realise, four decades ago, what he was getting himself into when he wrote the piece?

“I was a young man. You’re very secure in your beliefs when you’re a young man: you think you’re bulletproof,” he says.

He realised from the reaction of some musicians involved in preparing and staging the first performance, in 1969, that he was far from bulletproof. There were many sceptics and cynics among the classical fraternity who asked something along the lines of: “Who the hell does Jon Lord think he is?”

And, reflects Lord, it was a huge culture shock for the Royal Philharmonic, too. “They’d just been playing Brahms, conducted by Rudolf Kempe. Then suddenly they had this bunch of long-haired ’Erberts standing in front of them.”

And, though Lord didn’t know it at the time, there was a massive crisis of confidence behind the scenes, within his management, the record label and the publishers, who were worried that the project might turn out to be an embarrassing turkey.

Four decades on, Lord can see why. “I think the seed had been planted earlier by a recording, which I loved, of the Dave Brubeck Quartet with the New York Phil and Leonard Bernstein playing a piece for jazz group and orchestra.”

That was around 1965-66, just a year or two after Lord turned professional. “We formed Deep Purple in 1968 and at some point that year I realised we had the group that could actually do this idea, which had been there in embryo since that ­Brubeck/New York Phil experience.”

He ran the idea past Deep Purple. “They just said ‘yeah’ and went back to reading their Melody Makers.” And he put it to his manager, Tony Edwards, who was “dumbstruck”.

Edwards, he says, was “a ­­cultured man, and very decisive”. He came back to Lord, told him he had booked the Albert Hall and engaged the Royal Philharmonic for six months hence,

and politely requested that Lord got on with it and actually wrote the thing.

At the request of the ­publishers, Malcolm Arnold looked at the score (Lord does his own orchestrations), ­pronounced it playable and offered to conduct it.

“That generosity of spirit from Malcolm, who had a heart as big as the UK, changed my life.”

But Deep Purple, in at the deep end, faltered. “Only Ritchie Blackmore, the guitarist, could read music, and not brilliantly,” says Lord. “I had to teach them every note of it by rote.”

The concerto is in three movements and, in one sense, resembles the traditional Romantic model, with its tussle between soloist and orchestra (here between rock group and orchestra). The problem is that the amplified rock group and acoustic symphony orchestra are almost antithetical.

“That question of balance is at the heart of the concerto,” says Lord. “The piece is formed out of the exact dichotomy between an amplified rock band and a ­symphony orchestra.”

Lord is ruthlessly fastidious in pursuing an acceptable balance between the forces, without the group losing its edge or the orchestra being overwhelmed. “I’ve fought this battle around

the world for 40 years,” he says.

“I have to argue endlessly with people: do not amplify the orchestra too much or it stops sounding like an orchestra. It was a lot more difficult in 1969. Now we have the technology whereby you can amplify an orchestra quite well, and if you’re careful it will still sound like an orchestra when it comes through the speakers.”

But there is another dichotomy within the piece, he says, which can never be resolved until the full forces are together in ­rehearsal: the issue of tempo and timing.

“It’s an issue of orchestral time as opposed to rock-band time,” he explains.

“Rock bands work from the drummer, and the whole basis is that it’s constant tempo: once you start, that’s how it continues. Orchestral tempo is a completely different animal.

“It’s infinitely flexible and works to the principle of rubato, a supple, come-and-go sense of momentum. And it will change with different conductors, different orchestras and different environments. And it will change on the night. When the rock band is in, it’s up to the conductor to follow and keep the orchestra in time with the drummer.”

Lord is in a singular position. He doesn’t have to do anything he doesn’t want to.

“For a great chunk of my life I’ve been defined by being the keyboard player with Deep Purple, and I’m very proud of that. But I didn’t have a lot of time for extra-curricular activities. It was only towards the end of the 1990s I realised there was something else knocking at the door.”

So he now has his time for ­composition. And when requested to stage the Concerto for Group and Orchestra, anywhere in the world, he engineers it so that he can coach and use a young rock band from the region.

“I am having an absolute ball with this, working with young people and using local heroes in the piece,” he says. “It is such a privilege and an honour.”

Jon Lord’s Concerto for Group and Orchestra is at the Usher Hall, Edinburgh, Monday, ­October 5, 7.30pm.