Rock group Deep Purple meets Sir Thomas Beecham's august Royal Philharmonic Orchestra. An unlikely combination indeed, especially in 1969. But in that year they joined forces to premiere Concerto for Group and Orchestra by Jon Lord, who had founded Deep Purple with guitarist Richie Blackmore a year earlier. Jon himself was a keyboard man - to quote one reviewer: "Jon Lord tore the top of my head off with his ominous, neo-classical Hammond organ."

"Ominous, neo-classical Hammond organ?" Jon Lord laughed. "What a splendid description; I think I'll use it in the future!" We are talking in the music studio attached to his Henley home. In front of us, the score of the Concerto sits on a concert grand piano, where Jon is polishing it up for a performance that will shortly reunite him with the Royal Philharmonic, conducted by Oxford-based Paul Mann. How, I asked Jon, had he developed a dual love of classical and rock music?

"As a very young man, starting from about the age of five, my father put me on the piano. He loved orchestral music. So I grew up playing what you might call classical piano, and listening to classical music. We were given a piano by an auntie, who had married well and wanted to get rid of an upright, because she was going to buy a grand piano. My parents told me that I took to this piano rather quickly, and started banging out what appeared to be small tunes. In my teens, when rock and roll arrived, I fell in love with it. But fortunately I didn't lose my love of classical music."

As a result, Jon Lord harboured a desire to write a piece that combined rock and classical.

"When I mentioned this to my manager at the time, he did what you might call a pre-emptive strike: in the spring of 1969, he went ahead and booked the Albert Hall. And on the advice of a friend, he booked the Royal Philharmonic Orchestra, who happened to have a spare couple of dates in September. I said: Do you have you any idea what you've asked me to do, in only five months?'"

"At some point during that summer, my manager called the head of our publishers and said: How can I find out if Jon can actually do this without hurting his feelings?' The publisher had the idea of introducing me to the composer Malcolm Arnold, who was also on the company's books. So I went along, and met Malcolm, who was a great big bear of a man, a jolly man. He looked at my score, and after a 20-second pause - deathly silence - he said, with a big smile on his face: Yes, this will work absolutely fine.' I could hear breath being let out of lungs all round!"

Arnold not only approved of Jon Lord's concerto, he went on to conduct the first performance of a work that had to satisfy both rock and classical musical audiences: "Without emasculating either camp," as Jon puts it. "The orchestra had to be seen to be playing with sound and fury, gentleness and beauty, all that good stuff. The band likewise, had to be seen to be a rock band - red in tooth and claw."

All of which makes me long to have been a fly on the wall at the first rehearsal. How on earth did the sober-sided musicians of the RPO view the wild sounds and appearance of Deep Purple? "There was a kind of wary circling of each other," Jon laughed. "The first time we went to meet the RPO was for a photo-shoot. We went to Henry Wood Hall, where they were being rehearsed by a very establishment, wonderful conductor called Rudolf Kempe. To him, a rock and roll band was anathema. As we walked in there were wolf-whistles, as we had longish hair and all that kind of stuff."

"As for the first rehearsal, had it not been for Malcolm Arnold's presence and his advocacy of the piece, it would have been a disaster. Not surprisingly for the time, I would say that 80 per cent of the orchestra didn't want to be doing it - but the chaps in the kitchen department, the percussion section, were thrilled to bits. And I had a band that was intimidated beyond belief by this general attitude of the orchestra, and the difference in the way of looking at music. At one point Malcolm banged his music stand and said: Stop, stop, STOP! Ladies and gentlemen, you've got a band of young men here who are playing their hearts out, and you are not. We are going to make history tonight, so we might as well make music at the same time!' And that night they played their socks off."

Following that first performance at the Albert Hall, Jon Lord's concerto was played once more - by the Los Angeles Philharmonic at the Hollywood Bowl, no less. Then, like many another piece of new music, it vanished into oblivion - literally, for the score was lost. Then the story took a most unusual twist . . .

'I was in Rotterdam doing a concert with Deep Purple in the late winter of 1999 when I was approached by a young musician from the local conservatoire. He said: Mr Lord, I think I've managed to recreate most of your concerto. It's taken me two years, let me show you.' He took out reams of paper, and indeed he had recreated it from copies of the film and CD made at the original performance. He had perfect pitch, and if he wasn't sure, he had looked to see where the violinists' fingers were placed - or where the trombones might have their slides. It was an astonishing piece of musical detective work."

Jon Lord left Deep Purple at the end of 2002 to concentrate on composing - with much regret, in spite of the inevitable ups and downs that affect any group.

"It's a bit like a family, it has its great days, and its bad days - it's a bit like a marriage in fact. The Deep Purple guys are still terrifically good friends of mine, but I felt that I'd reached a natural break. I'd formed the band, it was my baby, and I'd played with it all those years, enjoying it enormously. But I'd always had this other side to me, and I realised that I was getting older, and if I didn't get some of this music written and hopefully performed, I might never be able to do it."

And talking of family, Jon Lord has one new composition already commissioned - one of his two daughters gets married next year.

"She's asked me to write something for the wedding: she doesn't want to walk down the aisle to Wagner, but she is happy to walk the other way to Mendelssohn - but to a recording with full orchestra, not just an organ!"

Jon Lord will perform his Concerto for Group and Orchestra with the Royal Philharmonic Orchestra on Sunday, October 7, at the Royal & Derngate, Northampton, as part of the Second Arnold Festival. Full details and tickets: 01604 624811 or online at www.royalandderngate.co.uk