Like Alexander Gibson before her, Susan Hamilton is a musician with a mission. As a soprano born in Scotland, she moved south for musical experience, and seriously considered settling in London - but returned at the appropriate moment to pass on her expertise to others.

Since she was in growing demand in London, and still in her twenties, it was a decision she could have come to regret. Her field of operation, as she knew, was more esoteric than that of the distinguished founder of Scottish Opera. Unaccompanied choral music - the art of a cappella singing that is one of her specialities - is a narrow taste compared with international opera. For such an ambition, London was an infinitely bigger pool in which to swim.

A decade ago, however, she fulfilled - indeed widened - her ambition through the creation in Edinburgh of the Dunedin Consort, an octet of voices who sing music both accompanied and unaccompanied, and will be celebrating their tenth anniversary this weekend with Handel's Dixit Dominus as a scintillating follow-up to their recent spate of Messiahs. These (one of them successfully recorded) have possessed a sharpness of focus more truthful to Handel's original than the big blockbuster performances that have long formed the norm, and still entail vast audiences dutifully rising to their feet for the Hallelujah Chorus.

When the Dunedins perform Messiah, as they now do each December, only the singers stand. Yet, refreshing though it is, it's not just this that has put them on the musical map. Nor is it the fact that intimate Messiahs, with the soloists doubling as the choristers, have become fashionable. What matters, above all, is the atmosphere they create; the sense of theatre Messiah should always possess but which was replaced in Victorian times by ponderous sanctimony. Susan Hamilton's singers bestow this on almost everything that they touch, whether it is baroque choral music in the concert hall or a staging of Menotti's Amahl and the Night Visitors in a Haddington church.

Hamilton herself started singing as a member of St Mary's Cathedral choir in Edinburgh, and she still speaks appreciatively of the grounding this gave her. But she wanted more, and London seemed the place to go. Soon she was appearing with the King's Consort, Philip Pickett's vivacious New London Consort and John Eliot Gardiner's racy Monteverdi Choir, which between them gave her experience of a sort she would never have got back home. Amsterdam, personified by the great Ton Koopman, beckoned. Philippe Herreweghe and his Belgian Collegium Vocale, Paul McCreesh and his Gabrieli Consort - all the right names in the world of choral authenticity - were soon seeking her services.

Yet, as she now confesses, she was not happy down south. Living and working in Scotland was what mattered to her and, with this in mind, she "wanted to start something". As it happened, the talented Ben Parry, now director of music at St Paul's School in London, had moved north. With him as conductor, she formed the Dunedin Consort, presenting a fascinating introductory programme in Edinburgh - "like a private view in an art gallery", she recalls - which set things on their way. All sorts of concerts followed, from English part-songs to Bach's St Matthew Passion and Monteverdi's Vespers. The Dunedins, in a short space of time, became part of the Scottish scene. How could we ever have done without them?

Then, ominously, things faltered. There were money problems, of the sort all Scotland's small musical groups were facing - and, in some cases, were being destroyed by. Parry, offered an appointment he was unlikely to refuse, moved back south. The John Currie Singers, who had established an annual chamber-sized Messiah at the Queen's Hall in Edinburgh, foundered. But just as Parry had arrived in Scotland at exactly the right moment, so did his successor, John Butt, who, having held several academic posts in America, became professor of music at Glasgow University. As a baroque expert, he was invited to conduct a one-off Messiah to see what happened. Could the Dunedins fill the gap left by the demise of the Curries? They could, and did.

Butt, says Hamilton, conducts selflessly, "bringing elements to the music we'd never believed possible". Though he may lack something of Parry's glamour, he is a musician of dedication, a profound Bachian - the Matthew Passion lies ahead - as well as a Handelian. As a result, Butt now shares the artistic direction of the singers with Hamilton - he conducting, she acting as lead soprano and "fixer", recruiting the necessary voices, arranging the dates, running the show. Bach motets and a new work by Peter Nelson will form part of the celebratory programme this weekend.

So what next? New works as well as old will continue to be featured. Among the latter, Purcell and Buxtehude (the tercentenary of whose death falls this year) lie ahead. So, perhaps, does Monteverdi, whose Orfeo was first staged exactly 400 years ago. But as a fearlessly adept exponent of modern music, Hamilton will keep on expanding her repertoire, in Scotland and elsewhere. Her voice can sound sweetly birdlike, eloquently forlorn, keen-edged as well as gentle. Henze's ravishing cantata Being Beauteous, which she is soon to perform with the BBC Philharmonic in Lancaster, should suit her to perfection. Let's hope she sings it in Scotland also. The Dunedin Consort celebrates its tenth anniversary at the Wellington Church, Glasgow, on Friday; the Whiting Bay Hall, Isle of Arran, on Saturday; and the Queen's Hall, Edinburgh, on Sunday. NB: these are not the original venues for Glasgow and Arran.