He lives quietly with his wife, Inge, who cooks, drives him to town in his Jaguar and patiently endures his terrible frustration. To be a great Romantic conductor, he says, it is necessary to have suffered (in which case Inge ought to be the greatest conductor in the world).
The view from his flat is magnificent. On wet days he stays indoors and thumps hell out of his Steinway, but on a clear day you can see all the way to the Danish islands. Ferries push out into the Baltic and warships slide majestically into the sound. When I visit, though, he makes me watch television - laser discs of his own concerts: Mahler Eight with the London Philharmonic, Mahler One with the Chicago Symphony, Beethoven with the Concertgebouw - over and over again, always the same.
It was 20 years ago, as a young producer for EMI, that I made my first record with Tennstedt. I thought I knew it all and didn't rate him much. He smoked like a chimney and I confess his conducting struck me as average. But I had to get on with it. We had nine Mahler symphonies to record for starters. He wasn't bad, but for me the real test of a conductor was not wallowing around in Mahler. I needed him to have a go at something really difficult - Beethoven or Mozart. Just clean, pure, classical music. Could he set that on fire?
Eventually we ran out of Mahler and it was not Beethoven but Prokofiev's Lieutenant Kije that gave me my answer. This wasn't his thing and I was worried sick. It was unfamiliar to the orchestra and needed the kind of absolute precision which I thought was beyond him. I couldn't have been more wrong. He took my breath away. I never doubted him again, and, as we produced one fantastic recording after another, I knew I was working with a great artist.
He hears the music so vividly in his head that he can use it as a yardstick against which to measure what he hears from the orchestra; from the first downbeat there is a magic which grips players and public alike; and, despite his poor English, he gets exactly what he wants.
How does he pull it off? His beat is imprecise, his gestures ungainly - players call him "the demented stork" - and he sometimes propels his baton into the orchestra. Also, he was born clumsy. If he can fall up a step, he will. Wine glasses are knocked over and the hems of long dresses stepped on. I should never have taken him sailing. I had imagined that someone sprightly on the podium would be fine on a heaving deck. But it was like having a lame giraffe on board and 40 yards out I had to rope him securely into the corner of the cockpit for fear of losing one of the world's most famous artists over the side.
Klaus trusts professionals, whether they're skippers, musicians - or firemen. We checked into a hotel at Glyndebourne once and, while he was having an afternoon nap, the alarm sounded. We all left the building. A head count revealed that the Maestro was missing. "Leave it to us," the biggest of the firemen said. "We'll get him down." Three of them rushed upstairs and returned five minutes later without Klaus. "There's this madman up there in pyjamas," said the fireman. "He wouldn't answer the door, so we had to break it down, and he says, 'You fight the fire, I'm sleeping'!"
That's typical. He believed in the fireman more than he believed in himself. Coming from East Germany, Klaus was unknown in the West and had to start from scratch. It was in the London Philharmonic that he found the players and management he could trust; and it was in London that he was to form his closest ties and make his name. I became managing director of the LPO just as Klaus was becoming a star and for the next eight years London heard some of the finest concert playing anywhere in the world.
Sadly, though, he's been plagued by ill health. He's had cancer, his ribs repaired and both hips replaced. He suffers dizzy spells when he flies. Nevertheless, he has always overcome affliction. He's a real fighter and still burns to conduct.
Meanwhile, most of the people in London whom he trusted have gone. With new faces arriving, it would be like asking him to start again - for the second time. One by one, his star players have drifted away: without him, they have one less reason to stay. Anyway, they're worn down by a system where the orchestras are owned by the players, who wield absolute power. It was a system designed to protect them but, as things get tougher, it's proving to be their weakness. No other city has a system like this.
We all miss him very much - audiences, critics and soloists, but none more than the players left behind in the London Philharmonic. They had the very best of Klaus and no one can take that away. We can only live in hope that, one day, he will return. Until then, I regret to say, London is a poorer place.
John Willan is Head of Music, BBC WorldwideReuse content