It is all there in the big hair and the big attitude. Steven Isserlis is a world class cellist. Now he needs his own pounds 850,000 instrument. Zanpe dri
hen the 38-year-old British cellist Steven Isserlis played as part of a trio at the Wigmore Hall in January, the crowd were thrilled. Perhaps this enthusiasm was unleashed simply by the music-making. But the talent of Isserlis aside, that night I was sure I was witnessing a star in the making. It was there in the tossing of the big hair, in the frowns and smiles, in the responsiveness of the player with his young colleagues.

Off-stage, Isserlis is no obvious Master of Cool. He has a sign on his cello case - "Rule Number One: I am never wrong. Rule Number Two: If I am wrong, see Rule Number 1." Behind the counter of a greasy spoon, or on a trainspotter's lapel badge, this would be unexceptionable nerdishness, but at Isserlis's stratospheric cultural level this advertisement of prattishness seems weird. He remarks that he's looking forward to seeing the Mr Bean movie and we seem to be locked into perma-adolescence.

It can't go very deep. A musician is a businessman, and must invest in himself. At Isserlis's stage, hovering between cult and celebrity status, his reputation needs careful management. This is supplied by his publicist, Ginny Macbeth, whose fees are on top of the upwards of 20 per cent his agent receives. Column inches shift CDs and put bums on seats. And talking of seats: being a cellist, Isserlis has to buy two airline seats when he travels. The instrument sits in its case beside him wherever he goes. It is a gleaming incubus which is much more Isserlis's master than his servant. He now often plays a 1740 cello by the Italian maker, Montagnana, which is mostly owned by a well-known ensemble player. Isserlis is intent on becoming the sole owner of this cello, at a cost of pounds 850,000. A trust of admirers led by a merchant banker and a financial adviser is attempting to raise the money as a loan. "I own 10 per cent of it now," says Isserlis. Soon this son of a music teacher and a university metallurgist (whose own father was a celebrated Russian composer) will probably be stumping up the price of a small house each year ("my outgoings will double") in order to pay back the loan on the main tool of his trade. No wonder he travels second class on the train and can only yearn for business class travel in the air. "Now, oddly, I'm under greater financial pressure than ever. I understand for the first time how it is that successful businessmen become bankrupt." Somewhere here is the surprising heart of an artistic career: the degree to which it is an enterprise, a gamble.

The main upshot of Isserlis's current and chronic expenditure will be that he has a cello with a bigger voice than his existing 1745 instrument by Guadagnini ("which is happiest if it's played gently"). This is necessary partly because increased fame brings bookings in bigger halls, and, according to one view, because Isserlis plays with animal gut strings, which make less noise than the more usual modern steel option. Isserlis's new cello can command every corner of the Albert Hall.

The cello's appeal is in its mature voice and understatement. Its perennial problem is to be heard over the orchestra. "A violin cuts through an orchestra more easily," he says, "like a child's voice in a crowd." The cello's is a voice built for mournfulness or for, at best, nuanced gladness. "You play like an old man," said Paul Tortelier when Isserlis was a teenager. But what use would boisterous youthfulness be to an up-and-coming cello player?

One reason why Isserlis will probably become such a big figure is that he is well-read and uses his knowledge in his sometimes tough discussions with conductors on matters of interpretation. One senses steel behind his judgements.

In his choice of the authentic gut, Isserlis compounds his reputation as a purist. The music journalist Helen Wallace has described him as having a "thoroughly modern stringency". She was citing his being a stickler for technical accuracy, and his refusal to pander to the crowd. This fits with the prevalent accusation that modern musicians are dazzled by technique but operate in an emotional vacuum.

"That's nonsense," says Isserlis. "The modern problem is hysteria, more like. People are inclined to think that bigger is better, and they pour intense emotion into every note. If an actor did that, he'd be laughed off the stage. The mistake with this is that there's nothing left for the emotional climaxes." That's what's so wrong, he thinks, with the "bleeding chunks" presentation of famous excerpts - there's no sense of the context of the "big" bits, none of the tension to which they are the catharsis, none of the quietude we deserve after the storm.

This is a very useful artistic snobbishness. It is just as well, however, that he is handsome as well as severe and articulate. His musicianship needs to be marketable. And it is. He has an RCA recording contract and is famous enough to be on a "Most Unforgettable Cello Classics Ever" CD (a groaning board of "bleeding chunks" if ever there was one) which boasts Jacqueline Du Pre, Paul Tortelier, Pablo Casals, and Mstislav Rostropovich.

Isserlis has teamed up with John Tavener, with his religious works - or gorgeous ramblings - which are inspired by the Orthodox tradition and which achieved vast fame when one of them was sung at the funeral of Princess Diana. Very crossover, very populist, I suggest. A tad ersatz, even? Not a bit, says Isserlis. Justifiably popular, that's all.

Isserlis's talk leads one to think that he understands nicely the deal that has to be struck between showmanship and ordinary competence. "Many people have star quality who don't think they have," he says generously. "And the reverse is also true," he adds with acerbity. Isserlis also acknowledges the degree to which sheer stamina separates the men from the boys. That and self-confidence.

But there needs to be particular character, too. One guesses that the strength of Isserlis's musicianship lies in the powerful tension he maintains between restraint, balance and intelligence, and emotion, expressiveness, and swank. He can be agreeably elitist round the edges: "I would love to be the Fischer-Dieskau of the cello but I have no desire whatsoever to be the Pavarotti," Isserlis told another interviewer. This is saying that a discerning audience at the Wigmore Hall will remain preferable to stadia-loads of the star-struck.

It was offers to play chamber music with his peers, and word of mouth in the business, that got him better work. He is proud of his partnership with the violinist Joshua Bell, and his teaching at the virtually private International Musicians Seminar in Cornwall. "I tell my students, `Don't do things to the music, let the music do things to you'."

Isserlis says that great music has plenty of room for performers to assert themselves, and that any sensible artist can find satisfaction within that discipline.

This autumn, there will be a wonderful opportunity to test the theory. For the third time this year, Isserlis will be playing Elgar's cello concerto. He is said now to "own" the piece as truly as did Jacqueline Du Pre in her day. In the hands of almost any player, the piece will transport people to the mists of a dawn in the Malverns in the evening of a man's life, for a discourse on ageing which seems to have hope, regret and anger. Is it fanciful to say that in Isserlis's hands it can take people even further into solace?

And as he plays, is it wrong to enjoy those masses of bubbly ringlets, greying round the edges, as they tumble with every toss of the head? As he flings his bowing arm about, scowls at demons, smiles at fancies, his expressions make a musical score in themselves

Steven Isserlis is featured in a Channel 4 film, `Schumann's Lost Romance', to be screened at the Barbican Cinema, 4 October. Late September, RCA release Isserlis playing the Schumann cello concerto in A Minor, which he performs on 3 October, 7.30pm in the Barbican Hall, London EC1 (0171-638 8891)