Mystery has long swirled around Thomas Adès. Britain’s leading contemporary composer doesn’t like giving interviews, doesn’t like being asked about his music or influences and, at least within the classical world, has “a bit of an attitude”. “I find it very difficult to talk to the music critics,” he admits, “because I don’t understand what they’re talking about. It’s much easier to talk to another normal person.”

Aloofness also has to be weighed against the weight of Adès’s precocious achievements and the expectation that came with them. He was runner-up in the BBC Young Musician competition as a pianist at the age of 19 in 1990. He received his first major opera commission at the age of 24, the critically acclaimed Powder Her Face, due to be revived at the Royal Opera House this year. He became the youngest winner of the Grawemeyer Award for Music Composition in 2000, and the Barbican staged a retrospective of his work last year.

Perhaps, however, age (even though he’s still only 37), experience and love are mellowing Adès. He is charming, full of laughter and happy to chat, even about his normally closely guarded private life. And maybe this openness is because his latest work is his first joint project with his partner, the Israeli video artist Tal Rosner, which has its premiere at the South Bank on April 28 – an ambitious piano concerto entitled In Seven Days, mixing Rosner’s hi-tech imagery (he’s best known for his striking title sequence for the TV drama Skins) with Adès’s score.

Adès and Rosner were among the first couples in the UK to enter a civil partnership in early 2006, within weeks of the new legislation being put in place. “That law came in quite soon after we met and I still can’t quite believe it’s there,” Adès explains. “I thought ‘we have to do this’ and not only because of the Israeli thing, although it did make life a lot easier with visas, but that was just a bonus. It has had an effect on my music. You feel more connected, like joined-up handwriting. I think my music is more joined-up now than it was before.”

Adès is not only unexpectedly happy to talk about his private life, but also his work. “I wanted to do something that would have visuals and tell a story but would also be a piece of music,” he explains. “I think of it like a ballet but instead of dancers you’ve got video.

“We’ve taken something that everyone knows, the Creation story, but as Tal is Israeli we took it from the Hebrew version, which is a little different. The story is like writing music; you start with chaos and it feels like it’s all jumbled up and suddenly you think ‘oh, these three notes – that’s an idea’ and then you develop them and that makes something else and by the end of the process with any luck you’ve got a piece of music.”

Rosner’s visuals will be played out across six huge screens with seemingly abstract images, taken from video footage of the two concert halls that jointly commissioned the piece – the Walt Disney Concert Hall in LA, represented by Frank Gehry’s swooping shapes, and the South Bank Centre (scaffolding used during the Festival Hall renovation masquerades as trees and plants on the third day of Creation).

Working with your partner is probably most people’s idea of hell, but Adès insists that the experience has been “harmonious”. “I was worried this was going to be an awful disaster,” Adès says, laughing, “but in fact it’s been incredibly natural because he knows my way of thinking and I know his. We did have a rule that at the end of the day we just had to say ‘no more’ otherwise the danger would be that you’d have no escape. You have to have something else to talk about.”

Adès’s musical tastes and influences are wide. When asked about the content of his iPod, he admits that there’s nothing classical among the 2,000-odd tracks. When asked to reveal its current playlist, he sheepishly admits to the 1980s Norwegian pop heart-throbs Aha, and says that he and Tal have been downloading nu electro from the Italian internet radio station Pig Radio. When I ask what music they had at their civil partnership ceremony, I get a surprising response. “We were up all night trying to figure out what the right music would be,” Adès smiles. “In the end we had Girls Aloud’s Love Machine.”

It’s another sign that he won’t conform to an easily classifiable stereotype. Back in 1997, his pulsing work Asyla depicted, in part, a night out clubbing in London. “Perhaps it’s because we’re married men now, but we haven’t been out clubbing for a while. Also, I’m 37 so I don’t want to be the embarrassing uncle at these things.” Adès sighs, then laughs. “There has to be a point where you say: ‘Am I ten years older than everyone here?’ ” Composing is not Adès’s only job. As well as conducting his own works (he will conduct at the Festival Hall), later this year he’ll conduct Stravinsky’s The Rake’s Progressat Covent Garden. Both conducting and composition also make way, on occasion, for the piano. So where does he feel most at home?

“I’m a composer on the dotted line because you have to be, it’s got to come first. The temptation is to stick to the conducting because – although I’m not saying it’s easy, because it’s not – in terms of the amount of time you have to prepare and the amount of money you earn it’s obviously the most attractive one. You could be totally seduced into doing it all the time and never writing any music, but that would give me a niggling feeling that I’m wasting my life. I know that deep down I’m a composer.

“On the other hand, when I’ve been at home for six months doing nothing except pushing notes around, I’m very lucky to be able to get on a train or plane and do something with other people, in public.” So much for Adès the recluse.

In Seven Days, Festival Hall (0871 6632500), April 28, 2008

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