COMPOSERS don't always have it their own way: there's a bit more to the business of writing music professionally than sitting down with an open mind, a sheet of manuscript (or electronic equivalent), a free hand and waiting for the Great Muse to arrive. It's not uncommon for them to be faced with instructions or apparent limitations set by the commissioner of their latest piece.

The five composers of Scottish Opera's Five:15 project, which premiered last weekend, were given strict parameters before their work began: pieces had to be no longer than 15 minutes, and the instrumental and vocal resources they could deploy were defined. Sally Beamish, in her recent commission for the SCO and the Rascher Saxophone Quartet, marched in to see the quartet, bristling with her jazz credentials, and was told: "No jazz."

How composers react and cope with the limitations of a commission is as much a test of their temperament as of versatility and experience.

John McLeod, whose own new commission for the Aberdeen City Music School, entitled The Leopard Dances, will be premiered this weekend, walked into a cracker when he received the invitation from the director of Aberdeen's specialist music school to write a piece for the young music stars of the north- east. "I was told candidly that they didn't have a full orchestra, that there were a lot of holes in it," reflects McLeod.

For starters, there were no violas and no double-basses. There was one trumpet but no other brass. And then there was a list of other instruments that might have had a less experienced or confident composer running for the hills. "The list I got included eight violins, three cellos, three flutes, three clarinets, one oboe and one bassoon, a load of percussionists, two guitars and two accordions. But there was also a clarsach, a quartet of singers and a piper."

The director's killer line, entirely predictable, was: "And I want them all in." At which point any remaining faint-hearts might have passed out.

McLeod, just turned 74, and one of Scotland's senior, if relatively unsung, composers, was unfazed. The only qualification he expressed was that the pipes should be left out. His request was granted. To some composers, the prospect of writing a new piece for such a motley crew of instruments and voices, and perhaps one of the most unorthodox orchestras imaginable, would be a nightmare. How can you balance such different instrumental characters? Who does what? How can you write a sustained piece that would be remotely coherent with such an uncommon grouping?

McLeod, as gentle and philosophical as he is about musical affairs, is also a pragmatist and the antithesis of the prima donna in temperament. "It is a limitation; but it's also a challenge. I just regard myself as a jobbing composer, and my attitude is: let's see what can be done."

He has also, in his armoury, the unassailable weapon of experience in working with mixed and unusual forces. Last year he received a Radio 3 commission which presented him with one of his biggest challenges: to write a single integrated piece for two orchestras, one professional, the BBC SSO, the other a youth group, the Highland Region Youth Orchestra. The result, a side-by-side composition entitled Fling, which fully integrated the members of the two into a monster, 180-strong orchestra, was premiered at the Aviemore Centre in a performance conducted by American Clark Rundell.

WITH the Aberdeen commission, the first thing McLeod did was get himself up to Dyce Academy, where the specialist school is based, to hear each musician, assess their capabilities and listen to their party pieces. "All of them are pretty good - they are all specialists - and soon I knew exactly what they could do."

His first decision was to stretch them by not writing down to them and by using, without dilution, his own musical style with its fractured rhythms and changing time signatures. He is also keen on dance music and dance rhythms. That preoccupation shaped the development of the piece, which evolved into a "fairly light-hearted" series of dances in the manner of a suite. The title derives from the coat of arms of the city, which features a leopard.

There is an overture, which, says the composer, is "quite tricky" and uses everybody. Then there are two songs. One, The Jeely Bap, is a setting of a humorous poem by William Soutar, and features the singers, guitarists and percussion, while the other, Aberdeen, uses a lot of Gaelic and has cadenzas for the clarsach, an instrument that, having no pedals, has to be written for with care. The two accordions come into the limelight in a tango; then there is a waltz; and, finally, the music erupts into a ceilidh, set as a dance in McLeod's style, with interludes for the accordionists, who play Scott Skinner tunes in the middle of it all.

McLeod, who will conduct the premiere, has enjoyed the challenge of the instrumental grouping. "I go to Hong Kong a lot. Every second shop is a tailor. Everything is absolutely made to measure. You couldn't buy that stuff off the peg. This is the same thing. It's a made-to-measure, bespoke, customised composition for specific individuals in specified groupings. It's custom built."

Once McLeod's new Leopard has danced in Aberdeen, the composer will down the baton and return to the neat, custom-built studio at his home on the outskirts of Edinburgh to resume composition on his next work, Haflidi's Pictures, a series of 12 or 13 piano pieces based on the paintings of cellist and fellow composer Haflidi Hallgrimsson, who is also a fine artist. These will be premiered at the Wigmore Hall in July by pianist Mark Tanner.

Before that, there will be two performances in London next month of his brass piece, Chinese Whispers, which will receive its London premiere on April 9 at St Bride's, Fleet Street, by the Onyx Brass, with a repeat performance in a Radio 3 concert on April 11 at Canterbury Cathedral.