ERROLLYN Wallen writes songs that have appeared on the same albums as music by Björk, Sting and Elvis Costello. She wrote a contemporary stage show, Jordan Town, which went down a storm at the 2001 Edinburgh Festival. The best of her many songs to date are contained in The Errollyn Wallen Songbook, published in London last week by Peters Edition.

So, you might ask, where's the logic in this weekend's unlikely collaboration between the cool Belize-born singer-songwriter and Edinburgh's foremost professional choral outfit, the Dunedin Consort - a group best known for its immaculate and stylish performances of Baroque masterpieces, complete with specialist voices, white-tie-and-tails and period instruments?

Things don't appear any clearer when you consider that this opening concert in Dunedin's tenth-anniversary season - a season thoroughly drenched in Baroque potboilers, from Handel's Messiah and Dixit Dominus to Bach's glorious St Matthew Passion - bears the misleadingly dry title Love and Death in the Lutheran Baroque.

Where, among the heady sophistication of Bach's exquisite cantata for soprano and bass Liebster Jesu, mein Verlangen and Heinrich Schütz's gracefully elaborate Vocal Concerti, could there possibly be a place for the moody, bluesy 21st century individualism of Wallen, whose other works include the quirkily titled Are You Worried About the Rising Cost of Funerals for soprano and string quartet, or All the Blues I See, written for flautist Emily Beynon and the Brodsky Quartet?

There is - as you'd expect from someone currently writing his umpteenth book on Bach and the Baroque - some logic in Dunedin director John Butt's programming. Its overall aim, says the Glasgow University music professor, is to illustrate the deeply sensual qualities Lutheran worship inspired among the foremost composers of the Baroque period. But just to spice things up, he thought he might throw in an example of how one of today's young composers might respond to the same impetus.

Thus, the unlikely commission for Wallen - a setting of words from the amorous Song of Songs, which so inspired Bach and his contemporaries. Wallen - whose relative mainstream obscurity is more to do with her unconventionality than anything else - jumped at the chance. "For a start, I got to set the rudest part of the Song of Songs," she says, referring to the sensual innuendos of Comfort me with Apples, a text she has scored for the same forces as the Bach cantata - essentially duetting voices (soprano Susan Hamilton and bass Matthew Brook) and period instrument ensemble.

But how does such a free-spirited eclectic like Wallen, who flits effortlessly between major Royal Opera House and Royal Ballet commissions and singing in night clubs, fit into such a stylistic straightjacket?

"I never think about style," she says. "For me, words and atmosphere are important, whether I'm writing a song or even as the inspiration for an instrumental piece. As a kid I was always vocalising, making things up. Isn't that what composition is all about? As a composer now, I simply write what I want to write. There has to be a sense of adventure."

Which is why Wallen's catalogue of works, dating back to the early 1980s, includes an impressive array of major stage works, music for radio plays, traditionally scored chamber music, operas, and even an organ work - Tiger - written in 2000 for John Butt, as well as the lighter songs that she frequently performs herself. And it's also why Butt, who describes her music as "more Debussy than avant-garde", was confident she could meet the challenge of writing for an ensemble of period instruments, among them Baroque violins and oboe d'amore.

Butt first encountered Wallen several years ago when he was teaching at Cambridge. "She came there as a Masters' student with a bit of a reputation as a pop artist," he recalls. Wallen knew she was a bit different from the normal Cambridge stock, but remembers Butt's advice. "He said 'don't even try to fit in'," she says.

What rubbed off on her most, however, was Butt's passion for Bach, an interest that later manifested in her first compositional encounter with period instruments, the song cycle The Queen and I, written in 2002. "Writing for that combination - Baroque violin, cello, harpsichord and me singing - was a revelation." The interaction of melody, bass-line and accompaniment, she says, helped her "really understand why modern funk music works so well".

It's that freestyle, joined-up thinking that makes the prospect of this new work for Dunedin so interesting. "Bach was the starting point," she explains. "I began working with his sound in my mind, then as I began to compose my own music, the two sound worlds began to collide, and I came up with something new."

It was equally important for Wallen to understand the soloists she was writing for. "I discussed things at length with Susan [Hamilton] and Matthew [Brook]," she says.

If anything is troubling her, it's the regret that she can't attend this week's premiere of Comfort Me with Apples.But Italy calls, where she has been awarded a residential fellowship to write music.

Foremost in her mind will be a new opera called Silent Twins that she is writing for Almeida Opera, due to open in London next summer. Based on the love-hate relationship of black twin girls, it could hardly be further from the rigid world of 18th century Lutheran church music.

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