The classical maestro has returned to his Argentinian roots -he's made a record of the tango
Up the hill from the Bayreuth Festspielhaus, overlooking a gorgeous prospect of rolling fields, the Burgerreuth restaurant carries an air of quiet conceit. Since the opera house is set in a bosky glade some way off the main road, and the only competition is a snack bar offering sausages, the Burgerreuth has a captive audience among rich and hungry opera-lovers (despite its name, it serves fancy Italian dishes) and can pick and choose its clientele. But should you mention that you're lunching with Daniel Barenboim, the languid, seen-it-all serving staff go into a rare old flap.

They fence off a table long enough to seat 10 people, should the maestro wish to sit at any end of it; they reserve another in the garden in case he should prefer that; they pull down a blind in case the sun might be too strong for his eyes; you have to sit where they tell you; they say, "You 'ave San Pellegrino? The maestro, 'e always drink San Pellegrino."

The maitre d' is nervous. The waiter is nervous. I am nervous. Mr Barenboim is not reputed to be a monster, but there are plenty of rumours about his asperity, his dismissive hauteur with the under-researched, the musically illiterate, the scandal-sniffer ("For God's sake, don't mention Jacqueline du Pre"), and the philistine. Discreet enquiries at his office revealed that he is happiest talking about "cigars, his relationship with the Chicago Symphony Orchestra, and general philosophical discussions". As I sit glumly wondering if it's possible to construct a conversation that simultaneously involves all three, the maestro abruptly appears before me. He is short (5'6"), dapper, good-looking, with huge expressive eyes and voluptuous Mediterranean features - though he has, of course, nothing to do with the area.

Since it seems the popular thing to be today, I ask: How nervous are you, considering you've got to conduct six hours of Tristan this afternoon? Barenboim roars with laughter (he has, of course, been appearing in public since he was in short trousers): "It's very bad to get nervous too early in the day," he says, wiping his eye. "Very tiring." What does he do on important conducting days? Meditate? Play golf? "It's very simple. If I'm hungry, I eat. If I'm tired, I go to bed. If I feel like doing some sport, I do some sport. I have no rituals. They're a bad idea. If you get used to drinking fresh peach juice at 3 o'clock in the day, what do you do when you're travelling, or it's winter and there are no peaches?" So, no Japanese girl in his dressing room feeding him raw fish before a concert, then? "Only in my dreams."

For a man who cuts such a grand figure in the international music scene - Music Director of the Chicago Symphony Orchestra, Artistic Director of the world's numero uno opera house, East Berlin's Deutsche Staatsoper, and visiting conductor all over the known world from Jerusalem to the Proms - he is surprisingly approachable, a confident and dandified figure. He is wearing a charcoal pinstripe three-piece suit, from which two huge panels have been cut and replaced by two shiny blue-grey silk panels. It's a very subversive look. "I got it in Spain, in a little shop in the port. You like it? It's not an expensive thing at all." Not that it would bother him unduly. He is, of course, extremely rich, supplementing his two salaries with high fees from his umpteen yearly conducting engagements that have all but overwhelmed his piano recitals.

What was the secret of being a maestro? Iron concentration? He frowns. "You must realise that sounds don't exist in the world. The fact that you can preserve them on a disc is an artificial matter. Tristan existed in Wagner's brain; it bore no relation to anything physical outside his brain. What he did, he used black spots on white paper as a kind of sign language for what he heard inside. But it's still only a sign language. So when you take this score of Tristan into the orchestra pit, you're literally bringing these sounds into the world. And you climb onto this wave of sound, you attach yourself to it and for six hours you live outside the time your watch shows you, in this sound-world of Tristan."

Barenboim can go on like this for ages, delivering slow, portentous, quasi-symphonic replies to simple questions. How you wish you could have encountered him when he was a child prodigy, learning the piano at five, giving his first concert at seven, knocking critics off their seats in New York, Paris and London before the onset of puberty.

There's a telling photograph of the Infant Phenomenon, aged about eight, his pixie face lost in contemplation of an orchestral score as other children might be absorbed by the Beano. Could he read a score as a kind of narrative? Could he hear the instruments as multiple sounds in his head? "Of course. Although it took me some time to master the art." Was he a very solemn little boy? Did he spend his time considering the grandeur of Bach's B Minor Mass? He waves the idea away. "I never thought in those terms. For me, music was always a very natural expression. And, of course, I was an ordinary child. I was a little devil. Not Dennis the Menace, perhaps, but not far from it."

One childhood trait he has retained is a collector's instinct. Barenboim is a determined completist: he's recorded all Beethoven's sonatas and concerti (with Klemperer), all Mozart's piano concerti, the whole Wagner canon, and now he's working his way through the Bruckner symphonies - as the Proms audience will hear when he performs the Eighth with the Chicago Symphony Orchestra at the Albert Hall on 12 September.

Two days after the Prom, Barenboim will take the orchestra to Manchester to mark the opening of the glitzy new pounds 42 million Bridgewater Hall; his fondness for Mancuniana dates back to his long-standing friendship with Sir John Barbirolli, of the Halle Orchestra, who turned all his annotated scores over to Barenboim when the latter took up conducting full-time.

When he studied a score, did he experience an urge to correct or re-compose it? The question elicits a strange double response. "Not at all. I'm only there to bring these sounds into the world," he says first. But ask if there's no more active role to play, and Barenboim's pride flashes a fin. "Of course in the end, I'm the one who has to judge the relation between time and space. To be audible, you have to start balancing what is important with what is less so. In that respect, every conductor has a very active role." But I thought you said you were a servant. "No, no - the servant is the one who brings the food from the kitchen to the table. Is the composer the one who cooks, or the one who does the shopping? I think the conductor and the musicians are in between the shopping and the cooking. Definitely in the kitchen."

Amid all this Cantonan braggadocio, it's interesting that he's just released a record of Argentinian tango music, entitled Mi Buenos Aires querido ("My beloved Buenos Aires"), on which he plays improvisational piano, accompanied by bass and bandoneon (a kind of accordion invented by a German called Band). Was it a blessed relief after the relentlessness of Wagner and Bruckner? "Nothing to do with it. When I was growing up, tango was very much part of the milieu; it was played on peoples' pianos at home. I grew up with it. I have a sentimental attachment to it, especially the music of Carlos Gardel. Whenever I went to Buenos Aires, I'd visit three or four places where they played tango on original instruments. Last time I was there, in September, I asked about the clubs and learned they were all closed. One because the singer had died; another had gone out of business. I was unhappy because I'd been looking forward to it. An acquaintance said, Professor, if you like tango so much, why don't you play some? So these musicians and I came together for fun, and we had such a wonderful time, we decided to make the record."

What appealed to him about the dance? The maestro pulled out a cigar the approximate size and colour of an overripe plantain and lit it for 30 seconds, as acrid stratocumuli of choking fumes billowed from both sides of his mouth like a cartoon rage. "It used to be danced like a duel by men with knives. Then it became the music of houses of ill-repute, a kind of bordello blues. What I like is, you have these free, decadent melodies and, underneath, this absolutely hard-as-nails rhythm which does not move. It gives you a feeling of destiny. All tangos are about destiny."

How Argentinian did he feel? Was there a residual gaucho in the Barenboim soul? "I don't know what it is about Argentina. When I go there, I don't feel like I'm going to a foreign country, but it's not home, either". But where on earth was home to him? His parents were born in Argentina, but their parents had been Russian Jews. He's always moved around - to Jerusalem, to London, to Paris, to Chicago, to Berlin. Where was his true identity? He considered it. "You know the tax authorities? When there's a doubt about nationality, they determine it as the place where you want to be buried. In my case it's Jerusalem."

London was famously home to Barenboim in the Sixties when he and Jacqueline du Pre were the classical world's golden couple, passionate collaborators and joint hosts of a north London salon, where starry international figures called Pinchas and Zubin and Itzak and Vladimir (dubbed "the kosher nostra") dropped by, en route to world domination.

Was it as idyllic as people make out? "We were just some people who played chamber music regularly," said Barenboim, sounding less than crazy about discussing these years. Had he ever felt English? "I did feel very much at home in London, but I never felt English, any more than I feel French in Paris or German in Berlin. My wife... " (his voice immediately takes on a wistful tone) "had some very English traits about her." What? The passionate, fiery, doomed Du Pre? Like what? "In her taste and behaviour. She was somewhat shy, in that slightly embarrassed way English people have. While another part of her was very international and un-English in its demonstrativeness." He sighed. "She was perfectly happy in Hampstead, even though she was already ill when we moved there."

Du Pre, as the world knows, contracted multiple sclerosis and died in 1987. Barenboim remarried, to the pianist, Helena Bashkirova. She lives in Berlin with his two sons, Michael, 11 (who plays violin) and David, 14, who plays electric guitar. "He is into rock and rap," says his father with fond distaste.

Lunch is suddenly over. The maestro has eaten a sirloin steak with gusto, harangued the waiter over the roasted vegetables, complained about the tardiness of the mineral water and talked, without ceasing, about how music and the emotions are intermingled - how, for example, it's useless to expect a child prodigy to conduct a funeral march with any meaning, how the experience of discovering sexual passion brings newly sensual elements to your playing.

We turned back to Wagner. How, I asked, could a Jewish musician conduct Die Meistersingers, with its explicit depiction of the town Jew as a talentless outcast? Barenboim launched into a long explanation of period context and "cultural nationalism" and concluded "I don't think you can make Wagner responsible for what the Nazis made of their vision of anti-semitism. And you cannot make modern Germany responsible for what their fathers and grandfathers did. But by the same token, you must remember you couldn't be a German nationalist in the second half of the 19th century without being an anti-semite."

It brought us to the final, crucial question, the one George Steiner called the central aesthetic crux of the 20th century: if we believe music has a moral value, how do we account for the concentration camp officials who listened to Schubert in the evening and continued exterminating Jews the next morning? How can the human heart be so split? Barenboim rose to the occasion. "I think the human heart has something beastly in it. But art can make it better. If you really live in the heart of music, not just live with it, it can teach you to see parallels. The Nazis you talk of, they couldn't see the parallels - the harmoniousness of the dialogue between the second violin and the viola in Schubert's String Quartet totally excludes these people because it is the principle of give-and-take, which they couldn't accept. If you let music become part of yourself, then it will change you."

And Danny the Boy Wonder, suddenly transformed into a wise old bird, shook hands and skipped off to the subterranean pit of Wagnerian angst