He may have written three or four of the ten best new orchestral pieces I’ve heard in the past 15 years. He may be professor of composition at Harvard, a remarkable accolade for a 39-year-old Englishman from America’s grandest university. But when Julian Anderson was asked to write a choral work for the Proms he went back to basics. He joined an amateur choir.

“I had to experience what choral music was like from the inside,” Anderson says. “So I joined the London Philharmonic Choir, and sang Elgar’s Dream of Gerontius under Mark Elder and Beethoven Nine under Kurt Masur.”

A revelation? “I was shocked by how difficult the Devils’ Chorus in Gerontius is,” Anderson admits. “You virtually have to memorise it to sing it properly. I also did rehearsals for Beethoven’s Missa Solemnis, which is terrifyingly difficult. Almost unsingable.”

Did that encourage Anderson to write even more difficult music for the amateur choir singing his Proms premiere, the excellent BBC Symphony Chorus? “Well, let’s say that I have naughtily made use of my inside knowledge. And although I am not a big fan of Gerontius, I did learn from that wonderful first chord of Praise to the Holiest (the climactic chorus in Elgar’s oratorio). There’s a big unaccompanied choral outburst in my work that releases tension in a similar way.”

It’s typical of Anderson that, even before the work is premiered, he’s already discussing its influences. Few composers are so self-assured about placing their work in an historical context or acknowledging their debt to the past. He even admits to taking out a notebook and jotting down the scoring of chords by earlier masters that have tickled his ear. “It’s not plagiarism,” he says. “It’s about breathing and living music.”

Anderson certainly does that. He has an encyclopaedic knowledge of music — doubtless one factor in Harvard’s decision to give him a professorial chair. “I was lucky,” he says. “My parents were very musical. My father, a scientist, had one of the first stereo systems in Britain. I can’t remember a time when I didn’t know The Rite of Spring.”

He studied composition at the Royal College of Music, but for a while his music seemed to be heading down the cul-de-sac of highly intellectualised serialism — the knackers’ yard of so many promising compositional careers in the 1970s and 1980s. “In 1990 I reached a watershed where I couldn’t compose any more. There was no flow, no momentum. My teacher, John Lambert, said: ‘You are living at crotchet equals 40’.”

Then by chance (Anderson is a great believer in serendipity) he saw something remarkable on television. “It changed my life,” he says. It was a celebrated Barrie Gavin BBC documentary about folk musicians in Eastern Europe in the late 1970s. The wild, modal music hit him with thunderbolt force. “In desperation to get my composition flowing again I took out my notebook, not knowing what I would write, and all these tunes influenced by Eastern Europe folk-music came pouring out. So then I started studying it in detail.”

Eastern European modes have galvanised Anderson’s music ever since. You can hear their influence not just in pieces that pay overt homage to ancient dance traditions, such as the brilliantly kaleidoscopic Khorovod (released this month on a fine Ondine CD of Anderson’s music), but also ingrained into the contours of works that have no obvious connection with that culture.

What Anderson had rediscovered was the power of melody — a musical quality much maligned by the late 20th-century avant-garde. “Everyone teaches counterpoint, but I don’t know one music college that has a whole course on melody,” he says. “Why not? It’s important, even if you’re not going to write melodic music, to understand how melodies were made in the past, whether they are Gregorian chant, Brahms or folksong.”

Anderson’s best pieces aren’t identifiable only by these twisty, melodic ideas. They also have scintillating energy and — even when a hundred things seem to be going on at once, as they usually are — a compelling sense of organisation. The “big line” carrying you through the piece is always there, though you often feel as if you are eavesdropping on some fervent primordial ritual whose meaning you can but dimly perceive.

His music has one other identifying feature. Nearly all of it is inspired by something non-musical: a painting, sculpture, poetry, some ancient artefact, or a combination of all these. For instance, his magnificent 2004 orchestral piece, The Book of Hours, was initially inspired by two medieval treasures. But its most unusual passage — in which the music of the opening is replayed through an electronic hiss, as if from an ancient 78rpm record — was influenced, Anderson says, by nostalgia for “those very badly pressed LPs of Polish music you could get at Collets in the 1970s”.

It is his ability to unify these highly diverse elements that makes his music so gripping. The new Proms piece, a big 35-minute score for mezzo, chorus and orchestra called Heaven is Shy of Earth, also draws its inspiration from different sources. Part of it sets texts from the Catholic mass. But this “half-Jewish lapsed Anglican agnostic” was never going to write a conventional religious piece. So the Sanctus is spliced with a short but teasingly enigmatic Emily Dickinson poem, Out of Sight? What of that?, which seems to suggest that even Heaven is wowed by Earth’s beauties.

At least, that’s how Anderson interprets it. “Actually, I’m not sure now that what I’ve made of it is what she intended,” he admits. “But who knows what she intended?”

Heaven is Shy of Earth is premiered on Aug 6 at the Albert Hall (020-7589 8212) and live on BBC Radio 3