A decade on, and MacMillan is still very much with us. A special South Bank and Barbican-based MacMillan Festival, Raising Sparks, begins on Sunday, including a few older works (Bsqueda and The Berserking receive their belated London premieres) but mostly featuring music written in the last few years. A couple of weeks ago, MacMillan drew up a list of the pieces he'd composed since the first performance of his opera Ines de Castro in August last year; it included a symphony, a clarinet concerto, a 20-minute piece for strings and percussion, a piano trio, some choral works...
At this point, his wife stepped in. "She's been very concerned over the last year, because I haven't been sleeping well. I decided it was time to recharge my batteries, so I'm going on a retreat to a Benedictine monastery. After that, I want to hear some more music. I think I've also got to learn how to smile at my kids again."
It's hard to blame MacMillan for this. Since the triumphs of the late 1980s, there has been a steady stream of tempting commissions. Would he have written so much music if he hadn't been asked for it? "Yes. Some of these pieces were written because I just wanted to. I wrote my Galloway Mass for the Catholic church in Scotland, some pieces for my daughter's school, because I just wanted to help out. I read Wallace Stevens's poem, `The Man with the Blue Guitar' and just set it. So even if nobody wanted my music, I'd still be writing it."
It sounds as if music simply pours out of him. "I compose very quickly, but when I'm composing, I'm aware that the ideas have been worried away at, sometimes consciously, sometimes subconsciously. Sometimes I work at more than one piece at a time; one is being planned while another is taking its final shape."
Critics and rival composers are bound to be suspicious of such facility - just as they were with Britten and Shostakovich. But there are other things about MacMillan that are likely to be controversial. If his approach could be summarised in one word, it would be "synthesis": modernist astringency combined with romantic folk-like directness; Catholic theology and left- wing politics with a strong Scottish nationalist accent. That's bound to offend purists, for a start.
"The range of reactions is wide, even extreme. One minute you get people wanting to throw their arms around you; the next minute they want to throw you in the Thames. You can never tell in advance what the reaction will be - even if you think you know where people are coming from. I find I get positive reactions from some so-called `conservatives' who are surprised how much they can engage with my music, while someone equally conservative will dismiss it as modernist rubbish. I remember the night Britannia got booed at the Barbican - I've still no idea whether it was the Hecklers, the reactionary element, or the New Music anoraks. Perhaps they were all sitting together."
A lot of the criticism in the New Music scene remains unspoken. People huddle together defensively in little groups and snipe at whatever's in the spotlight, or cultivate exquisite put-downs. But some of it gets into print. Does it ever bother him? "No. Well, not as it relates to the substance of the music. But when it gets more personal, if someone writes that my music is morally as well as artistically flawed - and there was a lot of that around Ines de Castro - well, my mother reads these things, and if she finds her son described as a pornographer in the papers, she begins to ask questions."
So to Raising Sparks itself. Officially, the festival, which takes its name from a new song-cycle to be heard on 5 October, begins on Thursday, but there is a gigantic curtain-raiser this Sunday when the LSO premieres the new symphony, Vigil, at the Barbican. There was always a chance that MacMillan would write a symphony one day. According to Mahler, this was the form that above all should "embrace everything". MacMillan has been throwing his compositional embrace wide for some time; nevertheless, Vigil is something of a milestone. "If you look at my earlier large-scale orchestral pieces - like Tryst and The Berserking - they're actually quite episodic, suite-like in a way. Isobel Gowdie is more through-composed. And it was there that I began to feel that the long-term paragraph, the long statement, was something I needed to work through.
"Even though the symphony can mean so many different kinds of things now, the idea of it as something long-breathed which tackles big issues like life-spans is what draws me. Vigil is the culmination of a triptych I've been writing for the LSO, based on the Holy Week sequence - this is the Easter piece, I suppose. The first movement is called `Light', but it's actually about 95 per cent dark. It's only towards the end that there are these flickers of light."
Religious imagery and extra-musical narratives figure prominently in MacMillan's own descriptions of his music. Are these the sources of the music, clearly identified in his mind before he begins, or things that emerge as he writes? "Nearly every piece I write has an extra-musical starting-point. That can be a point of controversy - you get cantankerous articles asking why no one is writing abstract music any more. For me, the connection between the pre-musical and the musical is unbreakable. The music has to exist on its own, but it would be a completely different animal if it had a different pre-musical stimulus. There's a transformation of one into the other - you could even call it a transubstantiation."
Does he insist on his pre-musical ideas, or are listeners free to hear what they will? "In a sense, they've got to be. People can encounter my music as pure music. But if they really want to get under the skin of it, they need to know something about why I wrote it in the first place. That's why I'm very careful now about the way I write programme notes. I can remember in some of my early efforts I used to rant a bit about Scottish politics and culture. Now I try to talk more objectively about the music. And yet, when you're talking about a symphony with movements called `Light' and `Water' - there are reasons for that, theological reasons - so people need to see what's emerging through the music."
Clearly it isn't simply a question of discovering a programme - words like "theological" suggest much grander ambitions. MacMillan may not be a musical evangelist, but he does have some kind of belief in a larger purpose for his work. Does he have a clear sense of what his music might achieve? "I have an embryo of a feeling about this. I feel there is a place for composers of serious music in the community - without being a social worker or a politician. I've seen and felt people's reactions - like when I conducted my Galloway Mass to a congregation of 3,000 people. I detect a thirst for deeper sustenance. I'm impressed by the way people from Russia talk about composers like Gubaidulina and Schnittke as prophets, as seers, who can disturb their listeners, and open up wonders. Contemporary music can't go on being the preserve of a small group of initiates. Everyone has right of access."
`Raising Sparks' opens at the Barbican on Sunday, with the premiere of `Vigil' (booking: 0171-638 8891), and continues at the SBC until 26 Oct (0171-960 4242)