Last year Peter Maxwell Davies was given six weeks to live. Despite ‘absolutely revolting’ chemotherapy and a blood clot that almost killed him, he went on to complete his 10th Symphony, and now he’s about to celebrate his 80th birthday

“I feel like I’m on trial,” says Peter Maxwell Davies, seated in the Royal Academy of Music’s penthouse flat for visiting professors. Davies, one of our greatest living composers, is about to celebrate his 80th birthday, is being feted up and down the country with a cluster of concerts at the Proms in London and in Glasgow on and around the big day. But here he is, feeling as if this epic encounter with his own music is something he’d rather not deal with.

“Seriously,” he says, with a glint in his eye, “all these celebrations are wonderful, but I did have to interrupt a piece I was thoroughly enjoying. I’m carrying it around with me in my brain – and I just want to get back to it.” By “back to it”, he doesn’t just mean composing. He also means his home on the island of Sanday in Orkney, the archipelago where he has lived since the mid-1970s. Whatever happens, he will be back there to vote “Yes” in the forthcoming referendum.

“I would like to see Scotland go independent,” he says. “Here we are, ruled by a bunch of Bullingdon-club millionaire Tories who don’t understand anything about Scotland. It’s very capable of going its own way, and one would get away from these dreadful right-wing politicians from all three parties here in London.” He pauses. “Well, perhaps one might, perhaps one might not.”

This new piece is presenting Davies with the latest revelation in a lifetime of creative discoveries. “Just before I left to take the boat to come over to Kirkwall to take the boat to the mainland, to take the train to Inverness to come to London” – Orkney really is another world from where we sit in Baker Street – “I was composing this sequence of notes, and there was a series of 32 four-part chords in straight minims [a regular, repeated rhythm]. And I thought: ‘I’ve never done anything like this before – what’s this all about? Go away and think about it, but I think you’re on to something – but I don’t know what, and I certainly can’t put it into words.’”

He’s lucky to be thinking at all. His is a musical odyssey that started in Salford in 1934, and that continued as a student when he was part of the influential Manchester School, alongside composer Harrison Birtwistle, their colleague Alexander Goehr, pianist John Ogdon, and conductor and composer Elgar Howarth. Last year he was given six weeks to live by doctors at University College Hospital in London, where he was treated for leukemia. “I’m on borrowed time,” he says, matter-of-factly. There was “absolutely revolting chemotherapy”, when he was given doses that would not normally be allowed for anyone over 60, because of his overall physical fitness, and a blood clot that nearly killed him. “I remember going under, and I couldn’t breathe, and it was horrible, and I thought: ‘Oh well, tough!’ I was quite prepared to go. They fought like crazy through the night to get rid of that.”

Music was a factor in his fight for life. One of the pieces the Scottish Chamber Orchestra will play on 8 September – his birthday – at the Proms is Ebb of Winter, written just before his leukemia diagnosis. “I find that piece quite disturbing. I was so pleased that spring was coming, and I wanted to celebrate that in the music. I didn’t realise when I wrote it what I was about to go through, but this piece seems to anticipate some of that experience.

Facebook Twitter Pinterest ‘Nature doesn’t concern itself with meaning: it just is’ … Peter Maxwell Davies on Sanday in Orkney in 2004. Photograph: Murdo Macleod

“I’ve often said that music knows something you don’t. Like my 6th Symphony: it knew that George Mackay Brown [the great Orcadian poet, a close friend of Davies’s] was very ill and was going to die. He died at the very moment I completed the piece. And I think that listening to something like Mahler’s 6th or 9th symphonies, you realise that the music knows what the author doesn’t. You can hear the jackboots of the first world war in Mahler 9, even though Mahler died in 1911, and you can hear all sorts of things, if you’re that way inclined – which I am – in the music that Schoenberg and Stravinsky were writing before the first world war, like the Rite of Spring.”

Throughout his treatment, Davies was at a desk in the hospital at 7am, composing his 10th Symphony for the London Symphony Orchestra. “And I knew that if I really did go in six weeks, I wouldn’t finish the bloody symphony. In the end it took much longer than that. The work was quite slow.” That’s hardly surprising, but Davies says that writing the music helped him through. Are there traces of his ordeal in the symphony, and in the music he’s written since? “I’m sure it’s there,” he says, “but I don’t think it’s a negative influence.”

Davies’s death would have been felt far beyond the LSO. His chaotic yet coherent masterpieces of the late 1960s, such as his Eight Songs for a Mad King, in which a violin is smashed to pieces every time the work is played – a moment that still draws gasps from any audience – through to his later cycles of concertos, symphonies, string quartets and music-theatre pieces,, as well as the dozens of pieces he has written for communities and amateur musicians to perform, make his a unique achievement in 20th and 21st century music. Yet his assessment of his life’s work is astonishingly, heartbreakingly modest. “I realise when I look at these people like Tom Adès or Jimmy MacMillan that they’re far cleverer and more brilliant than I could ever be, as conductors and in many other ways – but I kept my corner. And nobody else could write the music that I write.”

There are more important things than critical acclaim, he says. “My little piano piece Farewell to Stromness has almost become a folk tune. People just say, ‘I like that piece,’ and they don’t know who wrote it. It gets played an awful lot at funerals these days. And that’s very unusual, for a so-called serious composer, to write a piece that people like so much, and they don’t care who it’s by.” Anonymity in your own lifetime – the ultimate accolade for a contemporary classical composer.

Meanwhile, Davies’s thinking about his music seems to have acquired a new, mystical dimension. “My religious attitudes are very open indeed, but I do feel that for human beings to make a God in our own image is a terrible affront, because we’re so limited in our understanding.”

Of what? “Of everything: what goes on at a black hole or beyond, or what happens in the inside of a cell, or an atom. This is all so far beyond our comprehension that it can only be tapped through rigorous science, and we can only goggle at it because we don’t really understand how subatomic science works, and what meaning it has – if it has any meaning. Nature doesn’t concern itself with meaning: it just is. And it has been a huge joy and great privilege to have in some way tapped into that energy.”

So music, his music, is a shard of that bigger, mysterious creative force? “I think in any composer worth their salt, you feel that come through the music – you feel something that is absolutely transcendental.” That life-force is always present in Davies’s work, in everything from the grand populism of Orkney Wedding, With Sunrise, with its bagpipe-festooned finale, to the intense drama of the Fourth Strathclyde Concerto, two of the pieces the Proms will present. And it will be there, too, in the music he will continue to write on Sanday, in that house next to the beach on the island’s north coast, in his ninth decade.

• Prom 70: Sir Peter Maxwell Davies Birthday Concert, Royal Albert Hall, London SW7 (0845 401 5040), 8 September; Max at 80: A Celebration, various venues, Glasgow (0141 353 8000), 14 September.