IT was five years ago that I sat with one of Scotland's great modern painters in a cluttered, colourful studio in Cambridgeshire. John Bellany's studio at his home felt like a treasure trove. It was packed with canvasses, large and small, oil paintings of boats, of fish, of women and men, lobsters, shellfish and carrion. There were watercolours too. There were large model ships in the studio window, all marked with LH, for Leith. Memories of his upbringing in Port Seton.

It was a sunny day, and Bellany, although not in good health, was moving and lucid in his thoughts that day. He spoke about his illnesses – he had problems with his eyesight, with depression – but also about God. He said: "I just think if you have that guiding light, you will survive whatever comes your way, as I have done – all these transplants, pneumonia, three heart attacks, two strokes, all these things. I don't believe it was just me getting through these things, I think there was another influence pulling the strings. It might sound silly and stupid and people can say that I am silly, but that is always behind my way of thinking."

There was a fireplace at the end of the room, and it was cluttered with cards, notes, mementos and photographs, including one with Alex Salmond. Another was of his friend, and collector, David Bowie.

Bellany passed away in 2013, and Bowie, in 2016. Now the great painter and his friend are reunited again in a painting which has not been seen in public for two decades. The painting, of Bellany and Bowie, by Bellany, can be seen in a rather unusual show in London. It shows Bellany and Bowie, but with their best-known talents reversed: Bowie as an artist, Bellany as a musician, playing an accordion. In the background, through a window, is a lighthouse, and sea birds. It is entitled Bonjour Mr Bowie. It will be on show at the London shop, Fortnum & Mason, until October 28. The show at the store takes a look at Bellany's work across five decades. It features art from the Bellany Estate and the collection of Frank Cohen, a major art collector.

Helen Bellany, the artist's widow, spoke to me this week about the origins of the unusual Bowie painting. It is from the family's collection. It dates from when, after a show in London, Bowie asked the Serpentine Gallery whether he could meet the artist he so admired. Bellany at the time was terribly ill: he was awaiting his liver transplant which would transform his life, and his art. The visit from the rock star, artist and collector, however, was a delight, Helen said. "David wanted to see John, and of course John was fascinated to see David," she said. "It seemed that they immediately understood one another. He saw what John saw. He was searching in life for what John was searching for." She added: "There was an affinity between them. They had a wonderful time. David Bowie was a searcher, like all of us are. Searching for meaning in the world, interested in all the facets of life. And he had a great sense of humour, which John enjoyed and shared. It was weeks before John's transplant and was a wonderful meeting – he brought his son with him. They became friends. There's no greater feeling than when you meet someone who shares the same wavelength." Bowie went on to be a keen collector of the Scottish artist's work, and a friend.

These remembrances, and others, will be part of Helen Bellany's memoir of her life with John Bellany, The Restless Wave, which is due to be published next year by Sandstone Press. When we met five years ago, she had written 100,000 words. It may be shorter now, perhaps, but is likely to be fascinating, if at times difficult, reading. She says now: "He lived a lot in his life, but what a life he had. And what a person to share a life with. It was not always easy – it was difficult at times – but it was worth it."