Back in the 1970s, Sir Peter Maxwell Davies developed a rule of thumb.

If he was ever stuck for inspiration, he would simply stand on his doorstep for half an hour. "Seemed to work every time," he says with a chuckle.

It didn't hurt that Maxwell Davies - or Max, as he's known pretty much universally - was living on Hoy at the time; you don't need to venture far on Orkney's craggiest island to find scenery that will set the mind and the pulses racing.

Of course, Max did venture out beyond his stoop, striding across the high cliffs and fierce beaches to find his inspiration in the waves and winds and screaming birds of the North Atlantic.

The 79-year-old composer now lives on Sanday, a landscape that is softer but no less rich in its elemental soundscapes. He still walks his dogs along the shore and, after 40 years of writing music on and about the Orkneys, still finds daily inspiration from the land and sea around him.

"Every time I take a walk it's different," he says. "The sounds are constantly changing depending on the direction of the wind. I might hear the shingle on the beach from one side or the deep boom of the waves against the rocks from the other.

"There is also a small lake by my house that offers a completely different kind of sonic experience. It's an endlessly varied palette and I will never grow tired of it."

Max's way of integrating these sounds into his music has evolved over the past four decades.

"Obviously I can't just imitate the sounds in notation; that would be damn silly. It's more about capturing an essence. About capturing an almost spiritual sense of it all."

On Friday the Scottish Chamber Orchestra (SCO) premieres a new work by Maxwell Davies that, once again, takes its inspiration from the Orkneys. Ebb Of Winter is a birthday commission to commemorate the orchestra's 40th anniversary. But the name also carries a great deal of personal significance for the composer.

"It was written in January and February of this year," he explains. "That was a lovely period, very warm and calm, when you'd have thought that spring had arrived several months early. I'd never seen anything like it in all the years I've lived there. I didn't know that the worst was to come."

The composer was diagnosed with leukaemia in March and treated with chemotherapy in the following months.

Now he has made "as good a recovery as can be imagined," he says, and sounds in high spirits as we talk.

He's thrilled to be writing for the SCO again: it's an orchestra he has known intimately since composing their ten Strathclyde Concertos in the 1980s, and he remains their Composer Laureate.

Although "there are new members who I don't know personally," he says, "I approached Ebb Of Winter as if it was for the group of players that I knew so well back in the 1980s. With the Strathclyde Concertos I was consciously extending the soloists and also myself. When I wrote for clarinet or flute I felt as though I was almost becoming those instruments. I've written better for those instruments ever since."

Ebb Of Winter might be a birthday present for the orchestra, but it's no chill-back for the players.

There is more than a hint of mischief in Max's voice as he describes the score.

"What I admire most about the SCO is the sheer skill of the string players, and the fact that they can't hide anywhere. When I used to conduct symphony orchestras I often found a kind of scrappy antiphonal effect from one side of the stage to the other. None of that with the SCO: they're always utterly together, and that means I can really challenge them."

Add to that the fact that composer Oliver Knussen is conducting the premiere and Max was tempted to concoct something really formidable. "I felt totally uninhibited in the technical demands I could make of him," he says with evident glee. "He's capable of anything, and I've loaded him with trouble."

Max's output is staggering for the sheer scale of it: ten symphonies (the tenth premieres in London next year), operas, concertos, string quartets, music theatre, music for children, choral works, chamber works, music for state occasions - the list goes on and on.

Next year he turns 80 and shows no sign of stemming the tide. In fact, many of his methods have become smoother and faster over the years. "Things that I used to have to calculate - long isorhythmic patterns, for example - I can now hold in my head," he says. "Hopefully my harmonic language has become generally clearer, too. I do want to reflect my ageing in my music, but not with any sense of decay. With a maturing, maybe."

And he is still acutely aware of the social responsibility that a composer has within a community.

He writes music that is difficult and music that is deeply personal, but he also writes music that is useful. He says that the past year has made him "even more mindful of the turbulence that can suddenly engulf someone.

"I'm very aware of what goes on beyond me, what goes on beyond Sanday. There's a lot of injustice in the world. Hopefully music can be a civilising force."

The British music industry will of course rally to honour one of its greatest living figures for his 80th birthday next year, including that premiere of the Tenth Symphony from Antonio Pappano and the London Symphony Orchestra in February.

"But the composer also has his own plans for how he'll mark the occasion.

"There is a very special bottle of Highland Park that I've had for so long that I've really no clue how old it is any more. I was given it more than ten years ago, I know that much. I decided then that I would keep it for my 80th. And here we are already."

The SCO performs Ebb Of Winter at Glasgow's City Halls on Friday and Edinburgh's Queen's Hall on Saturday.