What’s bitten Andrew Marr? Perhaps it’s having turned 50 this summer, but on his Sunday morning show he has suddenly dropped the easy, breezy questions and gone for the jugular. One week it’s Gordon Brown and antidepressants, the next he’s accusing David Cameron of being “a wealthy man and a toff” who doesn’t understand the public.

Marr protests his innocence: “I honestly think that Gordon Brown and the pills was a natural and reasonable question, asked politely once in a 30-minute interview.” He only regrets that the furore overshadowed Brown’s admission that he made mistakes with the banking crisis. As to questioning Cameron about his wealth, Marr insists that “since he will be in a position, possibly — probably! — to tax the rest of us, where he starts from seems to me absolutely in court.”

Yet there is an irony in Marr, of all people, asking the more personal questions. Last year he took out an injunction preventing publication of details about his private life. Moreover the injunction was framed, like the oil trader Trafigura’s, to prevent even the mention of its existence. The move was designed, Marr says, to protect others rather than himself. But how much should the public be allowed to know about a public figure, which Marr now is?

“I think there’s no easy, hard-and-fast answer to that,” he says. “A world in which every aspect of our lives is public seems to me rather oppressive. And a world in which the people who control our lives are sphinx-like figures is also rather unattractive. Between the two there is a whole range of what should be private, and what shouldn’t. I don’t want to go into that — for obvious reasons — but I think it’s a reasonable debate and always will be.”

And yet, in the book accompanying his new TV series, The Making of Modern Britain, Marr happily prints a very entertaining chapter about the sex lives of Edwardian literary figures, not even politicians: E. E. Nesbit bringing up her husband’s mistress’s children as her own; H. G. Wells having an affair with one of those young daughters. Is that permissible because of the distance of time?

“Absolutely, yes. I think there is a commonsense thing here. The public interest and what the public are interested in are different things. And we all have a voracious appetite for knowledge about other people’s lives. It doesn’t mean we’ve always got a right to know about them. I think it’s about power and relevance. There are lots of things that are just irrelevant, even with someone like Gordon Brown. You know, his private private life — why would you want to know, and you have no right to know.”

Marr is married to the journalist Jackie Ashley, with whom he has three children. Yet he has become something of a lust object for discriminating women. This is baffling to Marr. He likens his ears to “red satellite dishes”. When asked for a single word to sum up his onscreen persona, he blurts out “goblin”. Helen Mirren is an admirer. “Aren’t those ears great?” she gushed recently. “I just want to grab them. He is like some fabulous little wonderful animal — and so incredibly smart.”

In his TV series and books Marr has the knack of taking complex issues and giving them a human dimension, extrapolating the great tides of history from individual ripples. In The Making of Modern Britain, the prequel to the very successful Modern Britain of 2007, which starts on Wednesday, Marr has been obsessed with finding “what person, where, at what moment: you are looking for fulcrum moments of change, something that expresses a bigger truth”.

The series covers the first half of the 20th century, to the end of the Second World War. So he travels to the beaches of Dunkirk, of course. But he’s more excited to discover, at Toughs Boatyard on the Thames by Teddington, “some of the narrowboats that went to Dunkirk! Still there!”

Marr’s eyes dance with pleasure, and he starts to wave his arms in the manner parodied on Dead Ringers. “So then you find there was an accountant, Basil Smith, and you bring it all down to this person, at this moment, deciding that for the first time ever he’s going to take his tiny little pleasure boat out to sea, and he sails straight towards the most appalling military catastrophe and gets 900 people off the beach.”

The horror and sacrifice of the Somme are encapsulated in Captain Martin, a former maths teacher, “who carefully worked out from drawings and models that he and his men would certainly be killed by enfilading machine-gun fire on July 1 and explained this to senior officers, but who nevertheless led the attack as ordered.” He and his 160 men were killed almost instantly.

“The last series,” Marr says, “was basically the triumph of consumer culture — how we stopped voting and went shopping. This series is our attempts to find interesting futures, political, ideological or spiritual, before we get to the shopping. It’s the stories of the Bloomsbury groups, the Roman Catholic idealists, the communists, the activists, the imperialists, all these people saying there’s a better way to live, we can reorganise society, it can be different. People then were more varied, disagreed with each other more than we do, had much bigger ideas about the future.

“But also it’s a period when we moved from being an empire containing a quarter of the world’s population to being a small welfare state off the north coast of Europe.”

Marr believes the greatest challenge facing politicians today is the impact of global warming: “On our security, on migration, on food, on the natural world, on how we are all going to live. And no politician will talk about it. I think there is a parallel with appeasement. In the 1930s, if you wanted to know what was going on in Nazi Germany, the evidence was there. But most people didn’t want to listen, they didn’t discuss it, it was far too nasty a subject.”

The “searchlights of scrutiny”, Marr believes, are a good thing. The BBC attracts more than most, but he wouldn’t have it any other way: that’s the price you pay for a licence fee. He loves the Beeb. He has rejected offers from rival broadcasters, and takes very seriously his duty to be neutral, “except in front of the shaving mirror”.

In the same spirit, Marr agrees with inviting the BNP leader Nick Griffin on to Question Time. “If you believe in democratic debate, if challenge and argument are how we move forward, you can’t exclude people from that. And it would be patronising to the viewer. People watching can make up their own minds. Television is a great revealer of character and personality.”

Marr is less optimistic about the future of newspapers. He was sacked as Editor of The Independent (twice) when he disagreed with cutting costs and selecting stories on the basis of what appealed to “vaguely liberal yuppies in and around London, since that’s what the advertisers wanted”. The industry has fallen on far harder times since. The only consolation, perhaps, is that now the press can attack Parliament over expenses without hypocrisy.

“I’m not clean in all sorts of ways,” Marr says, “but I am clean on that. I don’t claim expenses at all any more from the BBC. Not because I’m virtuous, simply because they’ve made it so unbearably slow, complicated, bureaucratic and intrusive. But if I go farther back, to my time in Fleet Street as was, it was completely corrupt.

“One of first things that happened to me as a trainee reporter was being hauled in to see the news editor, who said: ‘You haven’t claimed enough expenses, you’ll show everyone else up’. In a ghastly way, it was the same with the MPs and the fees office.”

Marr has made two cameos in Doctor Who. Is he a fan? Marr laughs. As a child, he watched it “literally from behind the sofa, not a figure of speech”. To this day, he says, the most terrible moment of his life was when he knocked for his best friend Giles — and the door was opened by a Dalek. “His mum had just given him a Dalek suit! And I can still remember the absolute . . .” For the first time, Marr is lost for words. “Nothing has ever been as bad since.”

The Making of Modern Britain begins on BBC Two on Wednesday at 9pm.

The Andrew Marr Show is on BBC One, Sundays, 9am. Start The Week with Andrew Marr is on Radio 4, Mondays, 9am

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