The eyes still sparkle. The wit and humour are undiminished. At 90, Nelson Mandela’s magic shows little sign of fading.

The body, however, tells a different story. Despite his iconic status, it is slowly revealing that he is a mere human after all.

His doctors have decreed that he is well enough to fly to London, one of his favourite cities, to attend the birthday concert in Hyde Park next Friday. This is almost certain to be his last visit to Britain and a poignant occasion as he bids farewell to his many admirers.

Mr Mandela walks painfully slowly; his feet are often swollen and bandaged. With the aid of a walking stick, it seems to take him an age to shuffle to his seat.

On his increasingly rare public appearances, his vehicle is brought as close as possible to his destination, usually a mounted podium or stage. A golf cart is sometimes used to cover the extra distance from the parking bay.

His eyesight, which was never good as a result of long incarceration in dimly lit cells, and hearing are inevitably failing, but not his smile and famous charm.

Doctors check him daily and often express themselves amazed at his fortitude. Some believe that the strength of character that drove him to survive 27 years in prison has allowed him to outlive all his contemporaries from the days of struggle against apartheid and to fight prostate cancer.

His fondness for Britain is partly due to the weekly demonstrations that took place outside South Africa House in Trafalgar Square throughout his detention. When he supported London’s bid to hold the 2012 Olympics, a move that was regarded as crucial in attracting support from Third World countries, he said: “There is no city like London. It is a wonderfully diverse and open city. I can’t think of a better place than London to hold an event that united the world.”

His minders, led by his wife Graça Machel, have imposed a news blackout on his activities before the concert. This is partly to avoid creating speculation about his health if tiredness causes him to cancel an event suddenly, and partly not to cause offence to those that he does not meet.

It is likely that Mr Mandela will meet the Queen, whom he calls “Elizabeth”. “He is probably the only man in the world who could get away with that,” said a former employee who heard him answer the phone with “Hello Elizabeth” rather than the more customary “Your Majesty”.

Bill Clinton, who attended a ceremony last July marking the beginning of a year of celebratory events for the former South African President’s 90th birthday, said that he used to dread receiving phone calls from Mr Mandela because he knew that he would have to do whatever was asked. “He would ask me if I was busy, and I would say, ‘Well, I am just running America’. He would then say, ‘Good, because I have something I want you to do’, and I would do it. That is this man,” Mr Clinton told the audience.

One thing is certain this week. Mr Mandela will not make a political statement nor give any interviews, in keeping with the decision taken when he retired fornally from public life a few years ago. He said then that he wanted only to spend time with his family, particularly his many grandchildren.

When he is not attending charity functions, he spends much of his time in Maputo, the Mozambican capital, at the home of his wife, who was previously married to the late Mozambican President Samora Machel. He is said to enjoy the warm climate there and also enjoys being less in the spotlight than he is in South Africa.

Plans for his funeral, expected to be one of the largest the world has seen, are never openly discussed, but all the main television networks have put in place massive contingency plans.

Rumours about his health are constant. The Nelson Mandela Foundation has to issue reports frequently saying that he is in good health. His birthday is actually on July 18.

Times recommends

• McDonald’s: the world’s local restaurant
• Help me to be a mum: a foyer to fulfilment
• A life online: God likes a laugh