All a question of keeping faith: A thief, a viola, and a hard choice: the composer Sally Beamish talks to Annette Morreau about paths to a vocation

What do you do if you get robbed? Take up composing? In the case of Sally Beamish, definitively. Neither the thief, the viola, nor the video has ever re-emerged, but Beamish took the event to signify that she should give up her profession as a viola player and become the composer she had ignored.

The robbery took place in London, June 1989, when she was highly active as a freelance player. Six months earlier, Beamish had composed a darkly sardonic work, No, I am not Afraid, a setting of six poems by Irina Ratushinskaya for reciter and chamber group. This work was to be pivotal; it reasserted in Beamish not only her faith as a believer, but her belief in herself as a composer. The trauma of the theft of the fine 1747 Gabrielli viola which was not even her own, but on loan, finally set the seal on her decision to leave the urban stress and distress of London, and head for the country of her husband - Scotland - and begin a new life.

Sally Beamish was born in 1956 into a musical family where she wrote notes before she wrote letters. At the age of seven she wrote an 'opra' based on a Paul Gallico short story. Her grandmother taught her to sight-read at the piano, but it was her mother, the violinist Ursula Snow, who encouraged (and discouraged) her interest in the violin, leading Sally at 15 to take up the viola. As a north Londoner, she 'mucked in' with the precocious band of chamber music players, as one of those useful musicians that could turn their hand proficiently to the violin, viola or keyboard.

At the Royal Northern College of Music, where officially she studied violin and viola, the principal, John Manduell, recommended that she attend the composition course given by Anthony Gilbert. Gilbert suggested that as an already well-formed composer, she should go her own way. But in the late 1970s, this way was not easy. Her work bore no relation to the densely complex music of the fashionable Darmstadt school. 'I was very angry and frightened by what I heard. I wanted to run away, but was sure that this wasn't what it was all about. I was threatened by it, but didn't have the confidence to realise that what I was doing was just as valid.'

Beamish stopped composing. She became a busy viola player holding positions of principal viola with the London Mozart Players and the Scottish Chamber Orchestra, and freelancing with the Academy of St Martin in the Fields and the London Sinfonietta. She now recognises this period to have been a very unhappy one, where despite her talent as a performer, she had nothing special to say.

A chance encounter with the Scottish composer Martin Dalby at the Brussels ISCM Festival in 1981 proved to be a turning point. Looking at her scores, he encouraged her once again to believe that she could be a composer. However, it was not until 1986 that she received her first 'professional' commission, from the Wilde Festival in Bracknell. Her panic was so great that as she came to attend the final rehearsal of Dances and Nocturnes, out came the ball-point and with it an attempt (resisted by the performers) to cut several bars, as she wrestled with the idea that her music could possibly be regarded as 'professional'.

Beamish entered a work for a competition at Snape. She didn't win, but some time later while playing with the London Sinfonietta on an Arts Council Contemporary Music Network tour, she met the composer Oliver Knussen who remembered the work. She acknowledges the invaluable help Knussen gave her while touring with British Rail, discussing her scores, Messiaen, and crises of confidence.

In January 1989, No, I am not Afraid received its first and (inexplicably) only performance by the London Musici to a tiny audience and no press. Anthony Marwood, violinist and leader of the newly formed Premiere Ensemble, was in the audience and was so impressed by the piece that he suggested to Mark Wigglesworth that the ensemble should commission a work. The June robbery of her viola was quickly succeeded by the news that the Arts Council had awarded her a bursary of pounds 2,500 for composition. It bought one year's child care for her five-month-old baby, and allowed her to write Commedia, the Premiere Ensemble commission, and to make the move with her husband to Scotland.

Commedia is a striking work and, like so many of her pieces, rests on imaginary imagery, in this case the tales of the Commedia dell'arte. Its first performance in London went unnoticed by the press, but in January 1991 in Scotland, at the suggestion of the composer James MacMillan, the newly formed Hebrides Ensemble included Commedia in its debut concert. The piece was met with ecstatic reviews, and Beamish was warmly welcomed into the patch.

Beamish's move to Scotland appears to have been an unmitigated success. With her husband Robert Irvine, principal cellist of Scottish Opera, and James MacMillan, she has founded the Chamber Group of Scotland. She speaks glowingly of the liveliness and energy of a gathering of composers and players generously backed by the BBC and Scottish Arts Council and enthusiastically promoted by the Scottish Music Information Centre.

A second child, and the lack of a publisher, have failed to abate the flow of compositions. She feels no sense of disadvantage as a woman; instead she appears positively to relish the discipline of having to compose (fast) during the few hours a day when the baby- sitter is present, while praising the limitless patience and support of her husband. Beamish appears serenely happy; during May alone, 10 performances of her works took place including a 'Soundbite' for English National Opera's Contemporary Opera Studio during the BOC Covent Garden Festival and a new Magnificat in Mayfest's Academy Now, somewhat unfortunately programmed alongside Peter Maxwell Davies's provocative Missa sur l'Homme Arme causing Beamish, as a believer, particular anguish.

Tonight, at the Queen Elizabeth Hall, the Scottish oboist Douglas Boyd gives the first performance of an oboe concerto, Tam Lin - the name refers to the ballad on which it is based - which is a second commission for the Premiere Ensemble. In July she is a featured composer at the Cheltenham Festival where another new work, Into the Furnace, commissioned by the Gaudier Ensemble for Schubert Octet forces, receives its first performance.

So what next? Beamish's 32 works have mainly featured chamber forces, although her Symphony (1992), commissioned by the City of Reykjavik, revels in the sound of brass and wind. A new violin concerto for Anthony Marwood and the BBC Scottish Symphony Orchestra will be premiered next year. But her real desire is to write an opera, and an opera is developing with the playwright David Pownall (author of Masterclass).

Based on an earlier play of his, Music to Murder By, it features a sequence of murders and hypotheses worthy of the attentions of Peter Greenaway. There's no commission for it. Perhaps an offer of compensation from the viola thief might be appropriate?

Premiere Ensemble, tonight, Queen Elizabeth Hall (7.45pm); Into the Furnace, 15 July, Cheltenham Festival (8pm)

(Photograph omitted)