"Lightning strike!" said Benn. "What's it about?"

"It's about God, sir," said the guard - and both men shared a smile.

Two years ago, Benn said he was "leaving parliament in order to spend more time on politics". He had entered the House in 1950, before half of today's MPs (including Tony Blair and Gordon Brown) were born - a confident, idealistic, leftwing voice: it was the year that Orwell died and Senator Joseph McCarthy started his communist witch-hunt. But if the train is at a standstill, the world has moved on, leaving Benn as probably the only person in the country who would instinctively assume that a lightning strike on the railways meant industrial action rather than inclement conditions.

"Hope is the fuel of progress and fear is the prison in which you put yourself," he says. And right now he is pinning his hopes on a privately run rail network getting him to the small, scenic Cornish town of Fowey in time to deliver his lecture, Free At Last, at the Daphne Du Maurier festival. The tickets were sold out within two days of being advertised, three months before the event. Nearly 600 people will fill the tent - more than three times the number who turned up to see magician Paul Daniels.

"We were shocked," says one of the festival organisers. "Not because we didn't think he would be popular, but because we had no idea how popular." The talk is part of a national tour, taking Benn from Glasgow to the Isle of Wight and many places in between. The response marks a national phenomenon. In a period when voter turnout has reached historic lows, public cynicism seems to be at an all-time high and political culture drifts relentlessly to the right, a passionate leftwinger performing socialist stand-up - cracking jokes about everything from Dyson vacuum cleaners and the House of Lords to Avon ladies and arms sales - has taken his act around the country and is filling theatres wherever he goes.

The lectures are only a small part of Benn's retired life - although he has been called on to repeat them at Harvard and Yale, and on Australian radio recently. Every day he gets around 50 letters and five invitations to speak. He spends a couple of hours reading the papers, still keeps his diary and has a column in the Morning Star. His weeks are packed with peace meetings, miners' galas, talking to sixth-formers and addressing rallies. When the meeting in Fowey was over, Benn headed straight back to London, arriving at around 11pm, having spent more than nine hours of the day on the train, so that he could be up at six to drive to Burford for a Levellers rally. He was on his way back to London to address a pro-Palestinian demonstration when his car exploded on the motorway. It is a tough schedule for a 77-year-old - a fact that Benn admits, but does little about: "It is so difficult to say no."

He has plans to work right up to the end. "The last entry in my diary will be: 'St Thomas' Hospital: I'm not feeling very well today'," he says. He even has plans for his gravestone. "I'd like it to say: 'Tony Benn - he encouraged us.'"

Herein, he says, lies the key reason for taking on the lecture tour - to motivate others in his retirement.

"I asked myself some time ago, what do you do when you're old. You don't whinge, you don't talk all the time about the past, you don't try and manage anything, you try and encourage people." It is particularly important now, he says, because the alternatives to the current rightwing drift are so rarely voiced. "At this stage, what's needed is a far greater understanding of what's happening and where power lies," says Benn. "I'm trying to encourage people to campaign for themselves and to make the interconnection between single issues. Single issues don't catch what needs to be done.

"When this organisation called me and said, 'Would you like to do a lecture tour?' I said yes. I had no idea it would work out like this." You will find his posters in the most unlikely parts of Middle England, next to posters for Marty Wilde, Joe Brown and Barry Cryer, "king of the one-liners".

The Maddermarket Theatre in Norwich, where he will be speaking in September, ran out of tickets in June. At the Little Theatre, Middlesbrough, they sold out only once last year (Shakespeare for children was on the bill); for Benn's arrival, influential local councillors who called for late tickets were disappointed. "I told a friend of mine recently: you said it would happen and now it finally has - I've sold out," says Benn, pointing to a poster bearing his face covered with a SOLD OUT sticker on his hall wall, and laughing until he coughs.

That he is such a hot ticket is partly thanks to his avuncular, approachable manner both on and off the stage. When the lights go up, only his pipe and flask of tea are in view - on a table beside a chair that could be waiting for Ronnie Corbett and his fireside quips. Benn ambles on, wearing a cardigan, checked shirt, braces and cords. He is one of the few figures who can extract a genuinely sentimental response from a leftwing audience. Big men with leaflets say they think he's "lovely". He turns militants to mush.

Moreover, having worked in parliament for half a century, he is, at 77, pretty much a national institution. His political life spans at least two generations; his political memory extends to the best part of a century. Ten years after he entered parliament, he began a long campaign to renounce his peerage following the death of his father, Viscount Stansgate, a former Labour MP who was made a hereditary peer. But he was both politically curious and connected from an early age. Ramsay MacDonald gave him a biscuit when he was five, he met Gandhi when he was six, bought Mein Kampf when he was 10, spoke on a platform with Nehru, was in parliament with Winston Churchill in the 1950s, debated with Malcolm X in the 1960s, was a minister through the 1970s, before spending two decades as one of the most prominent backbenchers and controversial figures in the Labour movement. When he unsuccessfully stood for the post of deputy leader in 1981, against Denis Healey, the conference was split almost exactly down the middle.

While some pointed to his refusal to toe the line under Neil Kinnock and Tony Blair as evidence of egomania that contributed to Labour's continuing electoral defeats, others praised him for trying to keep the party true to its principles. Harold Wilson once remarked that Benn "immatured with age". He is certainly the only politician to emerge from an Ali G interview with any kudos: "He took me in completely but I treated him with respect," says Benn, who is not above scattering some of Ali G's gags through his lecture. Those who once attacked him with fervour now try to crush him with their condescension. "He may have talked some blather over the years but it was always blather he believed in," writes Keith Waterhouse in the Daily Mail - a paper that once argued that Benn "imperils the economic lifeblood of the nation".

"That's the thing you worry about," says Benn. "What is the final corruption in politics? Earlier, it was to get into cabinet; before that, to be popular, but, later on, the final corruption is this kindly, harmless old gentleman... I'm very aware of that. I take the praise as sceptically as I took the abuse."

If his tour tells us a fair bit about Benn, it tells us even more about Britain. For while the lectures themselves are fascinating, those who attend them, and the reasons they give for turning up, are even more so. In South Shields Customs House on the banks of the Tyne, the audience was a working-class crowd. One man, his voice quivering, told how his young workmates were summarily fired one day: "Five men were called. They walked in, then they were walked to their lockers and walked out. I was ashamed that I didn't do anything, but I was afraid for my own job." In Fowey, where the Daphne Du Maurier festival pulls in a more conservative, older audience, one woman questioned whether "there'll be enough in the kitty for all of us" if "we keep letting in illegal immigrants and asylum seekers". Benn responded that we need more controls on the movement of capital than on the movement of labour. In Middlesbrough, where copies of the Weekly Worker and Socialist Worker were being sold at the door, one paper-seller greeted those arriving with the cry, "Blair, Byers, Benn - they're all the same".

The regional differences are less stark than the demographic similarities. Most audiences are aged between 45 and 55, with few young or black faces and a fairly even distribution of men and women, falling into three main groups.

First come the leftwing activists - either in single-issue campaigns or small hard-left parties - who seek to change the world, but whose daily experience suggests that the world is changing in ways they do not like, cannot prevent or are unable to fathom. The lone, sometimes lonely, strident, idealistic voices in the staff room, council chamber, residents' association or union meeting. The woman who travelled to Middlesbrough from a small village, where she is the sole anti-GM campaigner, picking up four friends on the way. The former Greenham Common activist who moved to St Austell, where she works with women at risk from domestic violence.

Then there are the Labour loyalists - lifelong door-knockers and envelope-stuffers, with an unbreakable attachment not just to an ideology but to a party that they feel no longer holds a similar attachment to them. People who were probably on the other side from Benn through much of the 1980s but, at some stage in the past 15 years of expulsions, policy shifts, U-turns and growing authoritarianism, today find common cause with him. Like the local councillors, who sit in Middlesbrough's health-food restaurant, Goodbody's, reciting the best lines from the previous night's lecture before heading for the council chamber. "The Labour party needs more people like him," said one party member as he left the theatre in Middlesbrough. "I'm to the right of him, but to the left of the party now."

And finally there are those who have no interest in socialism or the Labour party, but who like to be politically entertained, engaged and stimulated. Often older, more conservative people who recall Benn in his earlier years, never agreed with him and probably never will, but who recognise a keen mind and decent argument when they hear one. Ask them what other politicians they would pay £10 to see and two names keep coming up: Enoch Powell and Winston Churchill. "I don't have to agree with him to think he talks a lot of sense," said one elderly lady at the meeting in Fowey. "He believes in something and he's prepared to stand up for what he believes in."

Put these three groups together and they will normally find little to agree about. Put Benn in front of them and one broad sentiment emerges that gives them common ground - a profound dissatisfaction with the standard and state of our political culture. They complain that politics is no longer about ideas, democracy is no longer about choice and the media are no longer about information. When Benn says, "We've lost control of our destiny", it means something - although not necessarily the same thing - to all of them. Tony Benn is their unspun hero. "There are fewer and fewer people in politics who seem to have anything about them any more," said one woman in Cornwall.

Here lies the difference between antipathy and apathy. A recent BBC survey, Beyond The Soundbite, found that, while the nation was neither "de-politicised" nor "uninterested", it was "disillusioned" and "disconnected". When asked to finish the statement "I would get more involved if..." more than a third of respondents ticked either

"I thought my contributions made a difference" (24%) and "I thought anyone would listen" (12%).

Disengagement rather than indifference was the cause of the low turnout in the last election, according to a Mori poll commissioned after the event. Although 41% failed to vote in June 2001, only 11% said they were "not at all interested in politics". Sam Younger, the chairman of the electoral commission, said the results suggested there was still a high degree of interest in politics. "Voting is still seen widely as an important civic duty. The decline in turnout can be turned around because there is not an underlying lack of interest in politics." Bob Worcester, Mori's chairman, said the data showed that interest in politics had remained stable for more than 30 years. People voted despite the campaign, not because of it.

This is Benn's audience. It is also the far right's. Those who waver between cynicism and disillusionment, oozing all the while with resentment at the presentation and packaging that they feel has made politics more predictable and them more powerless. "The BBC had a rowing competition with the Japanese and lost," begins an anecdote that Benn likes to tell and audiences love to hear. "So John Birt set up a working party to try and find out why. They found that while the Japanese had eight people rowing and one steering, the BBC had one rowing and eight steering. The working party decided to employ consultants to devise a solution. They decided that what the BBC really needed was three steering managers, three deputy steering managers and a director of steering services. The rower, meanwhile, should be made to row harder. When they faced the Japanese and lost again, the director of steering services decided to sack the rower, sell the boat and give himself a pay rise." The punchline can barely be heard above the laughs.

It is a joke that strikes a bitter-sweet chord with a certain audience of a certain age. It is a swipe at a managerial class - the layer of personnel departments, human resource teams and consultants - that has expanded in the past 20 years in both private and public sectors. It is an attack on the jargon, targets, appraisals and inspections that have emerged to regulate our every working hour. It is wistful for a time when Britain produced goods rather than services, and when a promotion meant more money and more responsibility, rather than a fancier title and longer hours. Like much of Benn's lecture, it blends nostalgia, humour, common sense, ideology and idealism.

He makes a broad statement of democratic principles threatened by globalisation and spin, rather than a detailed examination of any specific issues. The talks start with a calculation: the number of people in the audience multiplied by their average age. "There are 400 people here, and if the average age is 40 we have 1,600 years of experience in this room," he tells South Shields. Then he lists five questions we should ask any powerful person: "What power have you got? Where did you get it from? In whose interests do you use it? To whom are you accountable? How do we get rid of you?" And he illustrates what he believes to be the three principal sources of power: money, faith and knowledge. After the interval come the questions, alternating at his insistence between men and women. This is his favourite bit. Benn loves the chaos, cut, thrust and interaction of the public meeting and mourns its demise. "The politicians don't want it, the media don't want it, and so the thing has been obliterated in favour of either propaganda or presentation. And neither of them has anything to do with discussion." The questions vary from the obvious - "What do you think is the most important issue of the day?" - to the unpredictable: "Do you think 17-year-olds should be allowed in the army?" to the challenging: "To what do you attribute the collapse of the Soviet Union?" Half a dozen tend to pop up wherever he is - Afghanistan, regional government, asylum seekers, funding of political parties, local government democracy and crime.

All in all, the lecture and questions tend to last around two hours. After an hour and a half of talking in Telford, Benn called for an audience referendum to see whether or not they should stop. They voted to carry on and he was stuck there for another hour.

The lecture ends with him calling for the audience to take charge of their destiny. "We have to organise. All this is about confidence and hope is the fuel of progress," he says, before quoting from the famous Chinese proverb: "But of the best leaders/When their task is accomplished/Their work is done/The people will remark/We have done it ourselves." And the lights come up, on his pipe and his flask

For further details of Tony Benn's lecture tour, call 01865 514830 during office hours.