BY any stretch of the imagination, the BBC SSO's City Hall concert this Saturday night is a truly extraordinary event.

The very titling of the programme, MacMillan conducts MacMillan, would alone justify the description: James MacMillan is now a seriously global figure in the world of composition.

But there is more to the programme than the title. The roster of music will feature two world premiere performances of pieces as yet unheard, a third world premiere of a new version of yet another composition and the long-awaited Scottish premiere of a work with some unusual characteristics. All of these are orchestral works, so a substantial, meaty evening lies in store.

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There is yet more. The two big world premieres, of works that form the bookends of the programme, are very early compositions, entitled respectively Symphonic Study and The Keening. They were written a long time ago and the composer, without any embarrassment, says he had "forgotten all about" them. They belong to a period just before his career as a composer took flight.

Somewhere, eyebrows will raise at that: how can a composer forget about music he has written, however long ago it might have been? Well, it happened, and here is the story.

Back in 1989, although he did not know it, MacMillan's music was nudging towards a launch pad. He had completed his first major orchestral work, Tryst, which was to catch the attention of the music world.

But the year after, in a Royal Albert Hall Prom concert, all hell broke loose at the first performance of his classic, The Confession Of Isobel Gowdie, after which the composer was greeted like a rock star as the full house went bananas and MacMillan was catapulted into the global firmament.

More of that followed with Evelyn Glennie's playing of the percussion concerto, Veni Veni Emanuel, and suddenly the world and its commissioners, its orchestras, soloists and choirs, all wanted MacMillan, new MacMillan, and as much of him and his music as they could get.

Since then he has never stopped and the demand for new pieces from him has never subsided. The man has not had time to look over his shoulder.

"The conversations I was having with the BBC about this concert ranged around the fact that, basically, I was discovering lots of pieces I had forgotten about in the wake of Tryst and Isobel Gowdie and everything that happened subsequently.

"Most of them were choral music. The scores were literally lying in drawers, including a Missa Brevis I had written when I was 17. There were loads of other little pieces from around then that I had just forgotten."

But in this phase of remembering the past, MacMillan's memory clicked on something else: The Keening. "I had written it the year before Tryst when I was studying as a post grad at Durham, and put it in my portfolio for my PhD. My idea was to try to find a performance for it but then things started happening quickly and it literally got forgotten about." Until when? Until after he had found the early choral music, which triggered the unearthing of other music from the period, including a short one movement symphony, now retitled Symphonic Study ("the original title was too prosaic").

What struck him particularly about The Keening, a very characteristic title, suggestive of "the ritualistic song of mourning in many traditional cultures", was that so much of it seemed prophetic of what was to happen five years later. "There was a lot in these works that pointed towards The Confession and Tryst, although The Keening has its own character and mood. It was technically proficient and I just thought I would love to do it.

"The Symphonic Study, nine years before Isobel Gowdie, is an even earlier piece from when I was an undergraduate in Edinburgh; it needed more surgery: you know," he says, with a laugh, "the sort of mistakes a 20-year-old composer makes, now sorted out by a 50-year-old composer. I had no thought of it being performed, but when I really looked at it, and sorted it out, I thought, 'No, there's a lot of this that's me'. I was pleased with it."

The other big element in the concert will be the Scottish premiere of A Deep But Dazzling Darkness for solo violin, orchestra and tape, jointly comissioned from MacMillan by the London Symphony Orchestra and St Luke's. The title is drawn from a quote taken from one of the metaphysical poets, Henry Vaughan: "There is in God, some say, a deep but dazzling darkness."

MacMillan describes the piece as "a kind of exploration of the impenetrability of the Divine". But there is another, and rather surprising association with the work.

"It was also written at a time I discovered that, before St Cecilia was patron saint of music, the music guilds of northern Europe venerated Job as their patron saint of music. This was very strange: this rather miserable and unfortunate character from the Old Testament to whom musicians brought comfort in the form of their music."

MacMillan has used an "unusually coloured" wind section for what he calls "a bit of a one movement fantasy for solo violin, strings and a colourful wind section" with deep instruments. The use of tape is unusual for him. "It is only the second time I've used tape in my music." (The other was in Parthenogenesis.) "It's taped voices, moulded and melded in a way that tries to build on the colourful orchestral palette. That is all I want the tapes to do: to create something the orchestra does not have, like a depth and a rumble in the lower level as an evocation of Job and the mourning."

The other two works are: For Sonny, "an intimate little piece written for and recently recorded by the Edinburgh Quartet", which MacMillan has rescored for full string orchestra in a new version the SSO will premiere on Saturday, and Exsultet, a brass quintet, and the only piece in the entire programme that has been performed before in Scotland.

So, a huge night for James MacMillan. "This is a very special programme for me, with all these premieres; and it is the first time for many years that I have worked with the BBC Scottish, and it will be nice to be back with them.

"It is also the beginning of an exciting year with them as we will be doing The Confession Of Isobel Gowdie at the Commonwealth Concert and a tour of India with the orchestra and Nicola Benedetti."

He never stops: he has just finished a new concerto for violist Lawrence Power and the London Philharmonic, is finishing his second percussion concerto, this one for Colin Currie and the Philharmonia; is off to New Zealand to conduct a new piece by Lyell Cresswell; to Brazil to conduct Marin Alsop's Sao Paolo orchestra; to America; to the Concertgebouw; and is also writing a piece for the RSNO and the RSNO Junior Chorus. Some career.

MacMillan Conducts MacMillan, City Halls, Glasgow, Saturday, 8pm