“I think of music like a religion and a science,” says cellist Steven Isserlis, sipping at a gin and tonic and waving his hands in big, wafty gestures that imply the point he’s making is really rather obvious. “Religion because the music has to be sacred, naturally. There is a genuine moral duty to do justice to what you see as the composer’s vision. Science because if you look at the score, if you really read it properly, it’s saying detailed things and you can deduce what the composer meant. It’s like detective work.” He looks up in anticipation of my next question. “And no, there is no tension between the two.”

It’s been a long day. Isserlis arrived from London via six hours of delayed trains then went straight into rehearsing Schumann’s Cello Concerto with the Scottish Chamber Orchestra for concerts around Scotland later in the week. When he emerges through the stage door at the Queen’s Hall — springy grey hair, alert, craggy face, grey cashmere turtle neck — he looks in no mood for an interview, which is understandable. How does he clear the grog of the East Coast Mainline to get in the right headspace for Schumann? We settled in a nearby pub, and finally he lets out half a smile. “Ah, well that bit is easy. I just have to hear the first few notes and I’m there. I’d have to be feeling really, really rotten to not respond.”

The long travel hours weren’t all bad. He used the down time, as he usually does, to write. As well as one of the world’s top cellists, 57-year-old Isserlis is an author who pens regular opinion pieces and has published two children’s books: Why Beethoven Threw the Stew and Why Handel Waggled his Wig, both of which have been translated into several languages. Now he’s working on a book for young adults. What’s it about? “90 per cent of my writing is about music and this one is about music, too. That’s all you need to know.” I try a different tack: why does he write at all? “Because I love writing. If I hadn’t been a cellist I would have been a writer. I love reading. I enjoy the process of writing — or at least, I enjoy revising.” He pauses. “Look, I’m not actually creating much when I write, I’m just describing. Describing music. I couldn’t write fiction.” Could he write music? “No way. I don’t have the urge. I couldn’t write music at anywhere near the level of the music I play, and that would be incredibly frustrating.”

Isserlis’s performances are intensely intimate, sensual, flamboyant. His trademark sound comes from using gut strings on a modern cello for the sweeter, grainier, more subtle kinds of expression they offer, and when 20 years ago an agent warned it was losing him work, that some orchestras wouldn’t hire him because of it, he declared it to be a non-negotiable musical decision and stuck to his guns. Which is typical, because Isserlis is a hard-liner of sorts as well as a romantic, never afraid to voice strong opinions, never shy of getting stuck into debates over musical politics.

“Musicians have an absolute duty to play new work,” he tells me at one point, “and any musician who doesn’t is letting the side down.” He himself has worked with Thomas Ades and John Tavener, and is keen for George Benjamin to write him a piece (“though I’m not sure he really wants to… I bumped into him the other day and asked him about it and he sort of ran away. Maybe I came on too strong!”). Yet he also admits that he is “lazy about learning new music. I lecture other people, but my heart is sort of in the 19th century. I love Schumann and Brahms. I love Victorian novels. Dickens, Wilkie Collins, Trollope, George Eliot…” Was he born in the wrong century? “Ha! No, I think hygiene is better now.”

This weekend Isserlis is back in Scotland to play Brahms’s Double Concerto with violinist Joshua Bell and the Academy of Saint Martin in the Fields. There will be no conductor — Bell leads the orchestra from the violin, which Isserlis says suits him just fine. “I say a lot in rehearsals, probably more than most soloists, right down to details of what the orchestra should be doing. I need to have a lot of input and be very involved, but I’m happy for the other person to make the gestures. It’s like chamber music: I have my ideas, he has his, and hopefully we synthesise.”

Besides, he and Bell go back a very long way. “Josh is like my brother. We’ve been playing together since he was 19, and that’s almost 30 years ago.” Isserlis remembers exactly what they played (Schumann’s Quintet and Ravel’s Piano Trio) that summer at Spoleto. “We were at a festival and everyone was talking about this young whizkid. I was determined not to like him but then we hit it off immediately.” So did Bell not behave like a whizkid? “Actually he did a bit, but I don’t hate the whizkid thing. I just hate the way people talk about it. Maybe whizkid is the wrong word: it implies he was just a prodigy with nothing to say, and Josh was never that. He is a deeply serious musician and always has been.”

The same phrase — ‘serious musician’ — comes up in reference to Robin Ticciati, principal conductor of the SCO. What does he mean by it? “Oh, just that they care more about music than about their careers. Not that they don’t also care about their careers: Robin and Josh — and me — we all care a lot about our careers. And of course earning money. I love big fees and whatever, and it’s nice to be a bit famous. I love attention. Good attention. I like to be recognised on the street, and I’d say that’s true of Robin and Josh too, they both love attention. But it’s not the most important thing. The music has to come first, which isn’t the case with everyone. That’s what I mean by serious musician. ”

Isserlis attributes his own seriousness to the Edinburgh-born Jane Cowan, who taught him cello between the ages of 10-17. “She was an idealist and she inculcated that in me. She made me feel I was friends with the great composers, and she made me feel music was a matter of life and death. She made me feel there was good music and bad music, good ways of playing it and bad ways of playing it, that it was a moral choice. That’s when I began to realise that music was like a religion.” But later, as he bundles his cello out of the pub and into a taxi, he pauses to revise his wording. “I think ’serious’ sounds too boring,” he says with a grin. “Maybe ‘real’ or ‘genuine’ is a better fit, because there’s so much humour in music. Beethoven: full of humour. Mozart and Bach? Hilarious!”

Steven Isserlis plays Brahms with Joshua Bell and the Academy of St Martin in the Fields on Sunday January 10 at the Usher Hall, Edinburgh