So far he has helped over 450 people commit suicide since founding Dignitas, the clinic for assisted dying, in 1998. Many of his 5,500 members are suffering from terminal illnesses, but others have non-terminal conditions such as osteoporosis, epilepsy or mental illnesses. Dignitas, or more importantly Minelli, believes that everyone has the right to choose to die.

Assisting suicide in Switzerland is not illegal, but it is punishable in Britain by a prison sentence of up to 14 years. Minelli has already helped 42 people from the UK to die, the most recent being Dr Anne Turner from Bath, who took her life in Zurich in January. She visited Minelli’s clinic accompanied by her three grown-up children. Her case was controversial because she was diagnosed only last summer with the brain disease PSP (progressive supranuclear palsy) and could have lived another three years. However, she was determined to travel to Switzerland before the degeneration of nerve endings, vision, balance and speech set in, and while she was still able to swallow the lethal solution of sodium pentobarbital that Minelli arranges for his guests.

Minelli has been called "the Howard Hughes of the assisted dying campaign" – he generally refuses to speak to the press because he feels that focusing on the dying process at Dignitas does not help his cause. But he did agree to speak to The Sunday Times Magazine because he says his work is about to take on a new direction. He wants to make the option of assisted suicide available to a far wider group of people – to the mentally ill and those with illnesses that are not in themselves life-threatening. He believes everyone should have the right to kill themselves, and that if you lift that taboo, the incidence of bungled para-suicides and painful suicides will decrease.

It is important at this stage to make the distinction between assisted suicide and euthanasia. In euthanasia, the doctor performs the last act in helping a patient to die. In assisted suicide, the patient must perform the final act themself, usually by drinking a lethal concoction. This is an exercise in free will that, in Anne Turner’s case, flouted both British law and the "natural" death awaiting her.

The busy Dignitas headquarters is on the floor below Minelli’s apartment, where six "collaborators", as he calls them, are fielding inquiries from suicidal customers. They are lawyers and other trusted supporters who have been hand-picked by Minelli to work with him. A death arranged through Dignitas costs 3,500 (about £2,450), which includes all the administration fees and payment to the collaborators. Travelling expenses to Switzerland are extra, but only a one-way ticket is required. Members also pay a modest annual subscription.

In his apartment, the quiet is punctuated only by the ringing of the telephone as collaborators call to consult him and let him know of new developments. Minelli says he is super-sensitive to sound and hates noise. He listens to peaceful music sometimes – "Definitely no Wagner."

As we talk, his latest client – a blind 92-year-old German veteran of the second world war – is committing suicide in the other Dignitas-owned apartment in Zurich. Minelli has already said goodbye to him. "I saw him here this morning. He was very calm, very quiet, and I wished him all the best. He will die within the next hour."

Minelli usually drives his clients over to commit suicide in the "dying room", which is a 20-minute drive away in the city centre. There, a doctor and another collaborator assist the suicide. Minelli himself is the controller, not the actual executioner – he says it is very important that he keeps a distance from the process.

Wearing an open-necked blue shirt, brown corduroy trousers and rimless spectacles, Minelli’s manner is both casual and provocative. He has a dark sense of humour and takes delight in memento mori like George Bernard Shaw’s "Do not try to live for ever. You will not succeed."

"I expect a call from the physician soon, to say the old man is dead," he says. While we sip our tea, the old man is putting down his white stick for the last time. A collaborator mixes 12 grams of sodium pentobarbital with a glass of water. The man’s last act will involve taking a drink from this glass. Then he will lie down, and after five minutes will fall into a deep coma. Twenty to 30 minutes later, he will die, just as I am being offered a second cup of tea.

Each time the telephone rings, I feel uneasy, knowing this could be the old man’s death knell. But Minelli seems unmoved: this is a well-tested procedure and there are rarely any delays or last-minute changes of plan.

He has no qualms today about sending another man to certain death. He is passionate about astronomy which, he says, helps him to put matters of his own, and his clients’ deaths, into perspective. "I have a 20-centimetre telescope which I mount in my garden. Sometimes at night the sky is ablaze with stars. I can see Jupiter with its four moons and Saturn’s ring system. I am familiar with the cosmos and I know this man is only preceding me by milliseconds, in a cosmological dimension."