Sure, all heroes look small close up. But Mikhail Gorbachev has, arguably, changed the world more than any living soul. So could this little man extending his hand really be the giant who ended the cold war and dismantled the “evil empire”? It is as if the loss of power has shrivelled him somehow. But any suspicion that Gorbachev might soon be stuffed and laid next to Lenin is blown away instantly. “Referendum, yes or no?” the last leader of the Soviet Union demands in broken English by way of greeting.

My colleague says he is against the European constitution so Gorbachev turns sharply to his interpreter. “You English. You love your pounds.” Ronald Reagan is dead and Margaret Thatcher is a bit gaga, but their old sparring partner — celebrating the 20th anniversary of his launch of perestroika and glasnost, his strategy to reinvent communism — is still entirely of the day.

Gorbachev launches into a long speech foreshadowing a lecture he will deliver in London (we shouldn’t just blame President George W Bush for our global unease: the world went wrong under Bill Clinton when he decided America could act alone; and the West needs its own perestroika to help the Third World). One is clearly meant to stare up silently into his magnetic eyes like a soldier saluting the Kremlin.

When I interrupt he raps me on the knee and his interpreter orders me to pipe down; John Humphrys would choke. When the snapper asks Gorby to look a little less thunderous, he says: “Why do you in the West always like people to smile in photos? Suppose I don’t want to smile?” But as he warms up, a mischievous smirk pirouettes around his lips. While telling me Roman Abramovich will have to return his wealth or face prosecution he adds: “But I do not want to undermine the motherland of soccer.”

Through the charisma one also glimpses loneliness. We meet in Geneva where he is feted at a United Nations conference. There are think tank groupies, motorcades and regulation Kremlin bodyguards: endless talks, but no pillow talk. When I ask how he has coped without Raisa, his glamorous wife who symbolised Gorbachev’s break from the USSR’s grey past, his eyes well: “You can say I am lost. The first three years (after her death from leukaemia in 1999) really threw me out of kilter. Raisa was always the foremost in my mind, and remains so in my memory. What Gorbachev is to the world, she is for me.”

He blames himself — or the coup that kept the couple under house arrest in 1991 — for creating the fatal stress. Yet he shakes his head: “She died when she had so much strength.” Clearly unaccustomed to personal questions, he gradually relaxes. He dispenses with his tie, chews on sugar cubes and grows ever more engaging. I ask what he made of the late pope and he discloses the two foes became such firm friends they corresponded until John Paul’s death.

“My opinion of him is very high. From the beginning we recognised he was different from other popes, but as he was fighting communism we saw him as an opponent. Communism was also a quasi-religion,” he smiles. “But things were changing. When I met him in 1989 we had a very good talk that started a dialogue that only finished with his death. He said he supported perestroika but he would also criticise capitalism when it crushed the human spirit.

“We agreed in all systems the human being should be foremost. Before his death he told me we need a new world order that is more stable, just and humane. To me, the pope was a great humanist.”

Again he smiles, revelling in this paradox. If communism is a faith, when did Gorbachev lose his? “I retain my faith in social ideals. And I think this shows my kinship with the person who started Christianity. To me, Jesus Christ was the first socialist.”

But he was not the kind of socialist who “repressed people and made democracy a sham, a decoration” — unlike Stalin, he means, who locked up Gorbachev’s grandfather for keeping a small amount of grain to feed his own family. “This led me, like Lenin, to realise we needed a different vision. I travelled the path. When I was in 10th grade I wrote an essay based on a poem, ‘Stalin is our military glory; Stalin is the enthusiasm of our youth’. Later I came to understand Stalinism had to be rejected, but it took a lifetime to come to that conclusion.”

Few Germans would want to sit in Hitler’s seat; after such evil the state had to be smashed and rebuilt. Did he feel queasy sitting in Stalin’s old chair? “No, I ordered a different chair, a revolving armchair, and it gave me a different view, different horizons.”

Intriguingly, Gorbachev paints himself as the first — and last — Blairite. “I believe in the kind of socialism Tony Blair is working towards. I regard him as a colleague.” In a sign that he is bolder than Blair, he says he has no problem with healthcare provided by insurance as long as everyone can gain treatment.

He followed our election closely and believes Blair nearly suffered the “disaster” of defeat. Did Blair show too much leadership on Iraq, placing Britain under America’s thumb against the wishes of the people? He laughs. “I have to say that is why he faced difficulties. The British people basically respect Tony Blair but sent a message they want their leader to rule in a more sovereign way. If they do give up some sovereignty, it must be pooled for collective action. It has been a lesson to him and the G8 that they must seek agreement. The problem is a lack of patience.”