One of the last things Tony Benn did as Viscount Stansgate, just before renouncing his right to a peerage in 1963, was to have a sample of his blood taken so there has always been a bit of blue blood in the house, a nod to the absurdity of the hereditary system. He is looking for it now, rummaging about in an old cupboard. “Aha!” he shouts, holding up a small plastic phial filled with a dark brown goo. “Blue blood. Bit clotted now, of course, but would have been worth a million in allowances.”

You have to laugh: it’s pure theatre. So it’s no surprise to find that straight after our interview Benn, 82, is off to Guildford to perform Writings on the Wall, a two-man show that he devised with Roy Bailey, an old friend.

“I read out revolutionary statements and he sings songs,” says Benn. About the Tolpuddle martyrs and suchlike. Benn claims that his audiences love it: “I say to them straight away, ‘You can relax, I’m not asking you to vote for me’, and you can feel the sigh of relief.”

This is a golden time for Benn, whose name was once a byword for the loony left – no wonder he is so good-humoured. He had the pleasure, last week, of seeing his 17-year-old granddaughter Emily, an aspiring MP and schoolgirl taking her A-levels next summer, speak at the Labour party conference (“I sat at the back and I burst into tears because I’m a terrible old sentimentalist”) and the almost greater pleasure of seeing a chastened David Miliband, the foreign secretary, talk about the “lessons learnt” from the Iraq war – a war Benn was passionately against.

He has just published the eighth volume of his diaries, which now add up to the most complete, if idiosyncratic, picture of British politics in the 20th century. This volume covers 9/11 and its aftermath, which saw Benn become president of the Stop the War Coalition and fly out to Baghdad to interview Saddam Hus-sein for a television company – something that he was, not surprisingly, roundly criticised for.

The diaries chronicle in minute detail Benn’s hyperactive work schedule as well as – touchingly and amusingly – the vagaries of old age. He is alarmingly frank about his deafness and the state of his prostate, but also movingly writes of his acute, ever present longing for his wife Caroline, who died seven years ago.

The clock on the mantelpiece in his rambling Victorian house in London’s Holland Park is stopped at six minutes past 10, the time she died. “I think about her every day, every hour,” he says sadly. “You can never fill a gap, the gap is always there, but you can decorate the gap with flowers and happy memories. I’m trying.”

Perhaps because her loss has made him more sensitive, these diaries and the previous volume, written while she was at the end of a long illness, seem far more human. The political rhetoric is leavened by stories of the arthritic old man he helped across the road who then asked, “Are you Tony Benn?” and when he learnt that he was, said, “I’d like to punch you in the face”, or his fumbling attempts to fix an electric bar fire that warms his bedroom. Although slightly embarrassed at being so candid, he is pleased with the result.

“If someone reads the diaries in 200 years’ time they’ll never have heard of Blair or Brown, they’ll be interested in a perspective on what life was like,” he says. “Like Pepys, you see. Pepys is full of famous people, none of whom I’ve heard of, but the theatres he went to and how he got on with his wife is interesting – not that I’m claiming to be a Pepys.”

His new-found sensitivity does not stop him duffing up Tony Blair, however – “Bush’s puppet” – whom he clearly despises (although he likes Cherie). Gordon Brown is a different matter. It is hard to be rude about a government when your son is a cabinet minister (Hilary Benn is secretary of state for environment, food and rural affairs). But even so he is surprisingly positive.

“He’s [Brown] one of the founders of new Labour with Blair and Mandelson so don’t make any mistake about that, but son of the manse and so on . . . He’s a person of integrity and that’s what matters in politics more than anything else. It isn’t ‘personality’, it’s whether you trust people and they stick to what they believe in, and he comes out well on that.”

It has to be said, however, that this benign assessment is at odds with an earlier view: in another of his diaries he remarked witheringly that Brown was “unfit to run a corner shop”.

He approves of Brown’s “big tent” though. “I’ve read that Tebbit and Mrs Thatcher and Quentin Davies and David Owen and Shirley Williams have all rallied round,” he says, smiling mischievously. “It’s a national government really, isn’t it? And Gordon’s the leader.

“I can understand why it’s happened, given the troubled times we’re heading for, because you can cope with Northern Rock but if it got worse and Lloyds TSB went bust you’ve got a problem. Because of my age I remember the 1930s and what that did.”

For that reason he thinks that Brown should go for a snap election: “It would be sensible because if this sub-prime mortgage turmoil goes on, people would want to feel there’s a safe pair of hands there, which he is.

“Funnily enough, in the summer of 1978 Jim Calla-ghan asked every member of the cabinet to advise him on when he should go for an election and I said ‘go now’ because the winter might be difficult and he didn’t and we lost. I’m not blaming him, but I think if he had gone in 1978 instead of 1979 he might have done better.”

Just think on that: no Margaret Thatcher and no endless arguments about whether Blair, Brown or David Cameron is the Iron Lady’s true heir. Benn met Cameron for the first time recently at the unveiling of the Nelson Mandela statue in Parliament Square. Cameron told him that he had read Benn’s book Arguments for Democracy and it had got him interested in politics. “I said, I take it you’ve never read Arguments for Socialism,” laughs Benn. “He said ‘No’.”

He can find humour in most things, but turning to the invasion of Iraq he is deadly serious: “The war was illegal, immoral and unwinnable and the unwinnability of the war is what’s registering now with the Americans.

“It helps to approach politics with a sense of history. Osama Bin Laden was paid by the Americans to get the Russians out of Kabul. Saddam was armed by the Americans to attack Iran.” Now we should pull out, he says, regardless of the consequences: “When Saddam went into Kuwait we didn’t say that he must stay and clear up the mess.”

There is much, much more in that vein but, suffice to say, Benn relishes his role as provocateur. “I am not a harmless, kindly old gentleman,” he says, “I got a death threat the other day and I was so pleased – I hadn’t had a death threat for ages.”

He is a charmer, though, and loves the company of women. One of the amusing threads in the new diaries is his friendship with Natasha Kaplinsky, the glamorous TV presenter who comes to interview him, then turns up with chocolates. Then they meet for lunch, which results in a tongue-in cheek story in The Daily Telegraph and a mocked-up picture of her in a ballgown and Benn in evening dress.

“I rang her up and said I hoped she didn’t mind but I’d written a very angry letter to the editor saying how outrageous it was to suggest it was a platonic relationship! That made her laugh.”

His great love was Caroline but his first love was his mother Margaret, whom he adored: “She was an extraordinary women, deeply religious. We used to read the Bible every night.

“My mum used to say the Bible was the story of the kings who had the power and the prophets who preached righteousness. She taught me to support the prophets.”

Which pretty much explains it all.

More Time for Politics: Diaries 2001-2007 by Tony Benn, is published by Hutchinson on Thursday at £20

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