Michael Nyman, sipping dumpling soup in a Japanese cafe around the corner from the museums of South Kensington in London, offers a delightful and unlikely image from when he became music critic of the Spectator in 1968 and his mother first realised he might make something out of all those wasted years studying music. "She would buy the Spectator," he says in his dry monotone, "and cut out my reviews and carry them around in her handbag to show to her friends."

It's like something out of Monty Python: Mrs Nyman sitting in the Lyons' Corner House with her circle, asking: "Have you seen what our Michael's got to say about Cornelius Cardew and his prepared piano this week?" It also fits nicely into Nyman's self-definition as the outsider of British contemporary music: the composer always ignored by the opera houses, scorned by his peers for doing all the vulgar things – having a large audience, writing film soundtracks, making money – that aren't done by those he calls "real composers". It's reached the point where he feels there's "something heroic" about his rejections from ENO and the ROH, "but it pisses me off, because I'm a very good opera composer, and I read with a combination of envy and joy when an opera that's been commissioned from my colleagues is given two stars by the Guardian. I know I could write an opera that's equally as bad, but I won't get a chance."

Nyman's sense of grievance about being deemed non-U by the contemporary music establishment is hardly new – the first time I interviewed him, for a football magazine, nearly 20 years ago, he observed glumly that he featured in more books about 1970s football than he did in books about classical music – but it's no less persuasive now, not least because he understands from both sides the pernicious effects of musical dogmatism. He cites the case of John Tavener, about whom he wrote for the Spectator.

"When John died, I thought maybe one of my reviews of his work could be used as a homage to his work in the early 60s. I reread this review, and it was terrifyingly savage." The irony, he says, is that all the things he professed to despise were things that "would really attract me big-time now, in that he worked with musical collage, and he had children's rhymes, and this very flashy incoherence, with pop connections. But in those days we were very hardline, and so his music was looked on with great suspicion. Now, if there was a big montage, collage piece, I'd love to write it myself. But because it broke all the rules of what the guiding musical language of the time was, I just dismissed the man. I have been cursed as a composer ever since I started writing in 1976 simply because the music was very consistent but it dealt with elements that were not really permitted in music, in the way I thought John Tavener was dealing with musical elements that were not permitted by the avant garde."

For all his outsider status, though, Nyman at 70 remains in demand. His latest commission – for the Liverpool Biennial – will be performed on 5 July by the Liverpool Philharmonic Orchestra, at the city's cathedral. Symphony No 11: Hillsborough Memorial features the mezzo Kathryn Rudge singing the names of the 96 Liverpool supporters who lost their lives at the 1989 FA Cup semi-final over Nyman's distinctive, metronomic, careful melodies.

Nyman has long been associated with football. In the early 1970s, he was part of a coterie of contemporary musicians – along with the pianist John Tilbury and composer Gavin Bryars – who went to watch QPR play, and he has one football-themed album, After Extra Time, in his discography. But there's still something odd about this commission. For Nyman has spoken often of music not being an emotional process but a matter of moving notes around on a page – and how can this piece be anything but emotional? "This is the exception that proves the rule," he observes, speaking slowly and evenly. "What you say is absolutely true: making that music, in the cathedral, played by that orchestra, with all those associations, can, and will, only have an emotional purpose."

Hillsborough Memorial has its roots in an earlier Nyman piece, 1985's Memorial, which was written to commemorate the deaths of 39 fans, almost all Italian, at the 1985 European Cup final between Liverpool and Juventus at the Heysel Stadium. The piece was performed just once, then four years later he began "reassigning" elements of Memorial, as he often does with earlier work.

On the afternoon of 15 April 1989, he was working in the studio on one of those sections, when he realised he was hearing nothing on his radio of the FA Cup semi-final score. He thought little of it until later, when he heard news reports of what had happened at Hillsborough, and became convinced he was to blame. "I had the thought I had brought it on, because the first time I took that music I had written for the Heysel disaster out of the box, another Liverpool tragedy happened. On the two occasions when I started playing with this particular set of material, there were not only football tragedies, but Liverpool-associated football tragedies."

Now Memorial forms part of Hillsborough Memorial, bringing the piece full circle, which is brave, given Liverpool's understandable suspicion of linking Heysel and Hillsborough. "It has involved a lot of talk in Liverpool. There have been a lot of sensitive discussions with people," Nyman admits. "But there was no way I could hide this iconic piece of music."

In fact, he says, the Michael Nyman Band performed Memorial in Liverpool recently. "And then, after the concert, my editor Mags showed me a piece of film that was shot in the concert. He'd shot a guy getting up in the audience and dancing to the music. He was then pretty roughly ushered out. I discovered after the concert that he knew Memorial, that he knew what its importance was, and he was trying to get the audience to join in some sort of cathartic social communication thing. It turned out he was a singer-songwriter in a group that did that Amy Winehouse song, Valerie." You mean the Zutons? "That's right, the Zutons. I don't think that happens in …" his voice trails off, but frankly you could insert pretty much any classical venue at the end of the sentence.

But that brings us back to emotion. If music is not emotion, what is it? He refers me to his stock answer. "It's pushing notes around a piece of paper." Well, I know Philip Roth says the only way to be a writer is to sit down and write, but … Nyman interrupts. "The difference between me and Philip Roth is that I choose not to write the musical equivalent of sex scenes. Or I choose not to write about my family life in music. So I can write music that seems to be totally autobiographical, because it's my music and no one else's music and I've created my own language and methodology."

Years ago, the Guardian's old email system allowed all journalists to see letters that were sent in for publication. Before we part, I remind Nyman about one he sent in complaining about a report on a game between QPR and Ipswich, which was never published, but which I – as a fellow QPR fan – remember vividly. He laughs. "That was so extraordinary! There was a QPR match that QPR won and played extremely well in – and the report didn't once mention QPR! Maybe that's the metaphor for my career."

Nyman on … The Cook, the Thief, his Wife and her Lover

"Peter Greenaway came to my house one day in 1989 after I had read the script for The Cook. And as I always do, instead of me saying, 'Well, I envisage this kind of music,' I asked the question: 'What kind of music do you think you want?' He described the piece of music as a slow, very processional, funereal pace. I stopped him in his tracks and got down a cassette from the performance of Memorial and said: 'Do you mean something like this?' He was startled and said: 'That's exactly what I want.' He said: 'Can you make me another piece like this?' and I said: 'No. Because if I make another piece like this, some or maybe all of the qualities you like in this piece might be destroyed.' He said: 'What should we do?' I said: 'Here's the cassette, take it.' So he then used that piece all over The Cook. I think the piece works very well in certain sequences. And in the sequence where he brings in the dead body I find it totally embarrassing. And horrible. I'm slightly embarrassed that a piece that was dedicated to the deaths of real people should be associated with a film such as that, but that was the decision I had made."

Nyman on … the music of his childhood

"There was no music in the family, and I was kind of picked out – I sometimes think at random – by a primary school music teacher who perceived some kind of musical talent in me. I don't know how or why. And he proceeded to give me this amazing musical education from the age of eight to 17. And so I went through everything – piano grades, music theory, harmony, what they used to call musical appreciation – and I don't remember rock'n'roll coming into it. In 1956, Bill Haley and the Comets came, and I was 12 and knew nothing about rock'n'roll. I suppose it was a class thing – I went to a grammar school, and most of my primary school friends went to secondary modern school. I have a feeling that the secondary modern kids knew about rock'n'roll and I knew about classical music. And my brother, who was two or three years older than me, was a ferocious jazz fan. So I knew about classical music and I was forced to know about jazz, because it was always in the house. But even when the Beatles played in the Odeon Walthamstow when I was in my early 20s I didn't bother to go."

Nyman on … finding his compositional voice

"Through the Portsmouth Sinfonia I met Brian Eno, when he was still in Roxy Music in the early 70s. I was a pianist but I only played classical music, and I said to him: 'How can I become a rock pianist? How do you adopt a style?' He was very wise then, as he is now, and he said: 'If you need it, you'll find it, and if you find it, you'll know you've found it.'

"In 1977, I had to give a concert in the National Theatre foyer and I had no repertoire. I remembered from when I was about 12 that there were 16 bars in a Mozart aria, the Catalogue Song from Don Giovanni, that I thought was harmonically absolutely amazing, and I had remembered it for over 20 years. So I got a score, got out these 16 bars, saw that the violins were going da-da-da-da, so I sat down and played the da-da-da-da like Jerry Lee Lewis. I never saw Jerry Lee Lewis, but suddenly there was this Jerry Lee Lewis gene in me that created this particular piece, this piano style instantaneously, as Brian Eno had suggested it might. And it has conditioned everything I have done since. I found my musical voice through medieval and renaissance instruments, through Mozart and through Jerry Lee Lewis. Which was not the path to being a composer that anyone could have foretold."

Nyman on … films and writing for them

"I very rarely go to the cinema because I hate acting. And the more films I watch – and most of them are on tiny screens on BA flights to and from Mexico – the more I'm conscious of the act of acting, and the act of a script having been written to put in the mouths of actors, and that just turns me off. And another reason I don't watch films is that I find almost every soundtrack intensely annoying. I've obviously had the good luck to write an iconic theme [for The Piano], but just as being a film composer has damaged my reputation in the classical world, so writing iconic film themes has damaged me as a soundtrack composer, because no one employs me. So here I am dragged out of retirement on to the stage in Cannes [to honour Jane Campion earlier this year], and all the film directors are telling me: 'Very nice to see you, Michael.' Well, I'm available, I have time on my hands, I'm not expensive, I'm easy to work with – well, that might not be true, but when I'm easy to work with, I'm really easy to work with. OK, here's a roomful of people who have the power not only to approve of my work, but also to increase the value – I'm afraid – of their work by calling up. But they don't. So I'm a retired film composer like a retired opera composer."

• Symphony No 11: Hillsborough Memorial is performed by the Liverpool Philharmonic Orchestra at Liverpool Cathedral on 5 July. In addition to the live performance, a recording of the symphony will be played in Liverpool Cathedral at 3.06pm on 6 August, 25 August, 3 September and 17 September. Details: biennial.com