George V did not marry an English woman in 1923 as we said in the interview below. In that year his son, the future George VI, married Elizabeth Bowes-Lyon who, though born in England, came from a Scottish family. The quote in the article below: "When I hear the word culture I reach for my revolver," is usually attributed to Goering, not Goebbels.

There is not much reconstruction in The Windsors, the last in David Starkey's epic 17-part television series on the British monarchy; as the Windsors coincided with the advent of film, and then television, there generally doesn't need to be. The one recreation is of a 1950s living room, very like the lower-middle-class Kendal one in which Starkey, aged eight and in his Sunday best, encountered television and monarchy for the first time. And, he notes with great solemnity, "I've never forgotten it."

It was the power, of course, the sheer sense of national event, allied with new technology. "But it was also - it was actually very very touching. You had this huge" - Starkey must be persuaded to knot his hands together or something when he's on television; in person they do much of his punctuation and illustration for him, weaving, and fluttering, and carefully making shapes of paper napkins - "apparatus of bishops and nobles and generals and soldiers - male, male, male, male, male. And this one woman in white. The impact of it. And of course, she was crowned alone. Philip merely paid her homage."

This week Elizabeth II surpassed Queen Victoria to become Britain's longest-living monarch. Her annus horribilis seems rather a long time ago now; the royal family seems largely to have recovered from the vilification it incurred at Diana's death. And yet, as Starkey argues, simple endurance is not everything; in fact constancy, in the way the Queen interprets it, may well be her downfall: if the monarchy fades away in the next couple of decades, she will, he believes, have played her part in this.

The argument is rooted in his characterisation of the Windsors, and the circumstances of their self-creation as a family in 1917, when George V, Elizabeth's grandfather, saw that the continuing existence of the monarchy was threatened by the combination of the Parliament Act and his continental alliances, and took the bold step of ditching the latter. He also remodelled the honours system (allying the crown with social and public, rather than purely chivalric, service); decreed that his family would be called Windsor, and that Windsors could marry Englishmen and women; in 1923, he married an Englishwoman himself. "You cannot resolve that [your marriage] shall be happy," said the officiating archbishop, "you can and will resolve that it shall be noble." "Windsor" became synonymous with public service, ceremony, duty, family values - and Elizabeth, of course, has striven to be a perfect exemplar.

Being a Windsor has been both a crutch and a liability. We talk about the Queen's entrapment between tradition and lachrymose modernity when Diana died. "I think there's no doubt whatever that the Queen herself simply froze. My view is that she has always tended to use protocol as a mask ... You may feel small, panicked and terrified, but if you have that mask ... " - he mimes a courtier snapping to attention.

"I think that in many ways she has found her role often very frightening, often bewildering, and requiring her to do things in which she has absolutely no interest, and to meet people she finds deeply unsympathetic, like me." Why him? "Well - I don't think she's at all comfortable with anybody - I would hesitate to use the word intellectual - but it's useful. I think she's got elements a bit like Goebbels in her attitude to culture - you remember: 'every time I hear the word culture I reach for my revolver.' I think the queen reaches for her mask."

One could be mischievous and say perhaps she was reacting to something else she knew about him; that, at the tail end of nearly 30 years of middling-successful academe (at Cambridge and the LSE), he discovered the joys of being an "all-purpose media tart" (his words), and his eviscerations of guests on Radio 4's The Moral Maze earned him the tag "the rudest man in Britain". Or the fact that she will have known he has made his name, and a reported fortune, anatomising her ancestors. Starkey last met the Queen when she was awarding him a CBE. He told her his next subject was ... her. "She looked a bit blank, so I said, 'It's quite complimentary.'" On Boxing Day she will be able to judge for herself.

Her lack of curiosity extends, he thinks, to the history of the monarchy itself. In 2003 Starkey was asked to curate an exhibition on Elizabeth I at the National Maritime Museum. After it was hung, he had lunch with the Queen. It was not a success for either of them. She was piqued - as he tells it - because her gin and Dubonnet did not arrive quickly enough; he was piqued because she showed no interest whatsoever in his show. It took him a while to work out why. "I didn't realise that she had no interest whatever in her predecessors. Her history begins with her great grandfather. That is it. It really is it.

"Nobody with two brain cells would dream of reading all the Christmas broadcasts - I have done, and it's quite fascinating, because her frames of reference to the monarchy, despite this 1,500-year history, are entirely her father and grandfather ... There is a reference to Elizabeth I. It was in the second Christmas broadcast when - I remember vividly - there was all this talk about a second Elizabethan age. And Elizabeth turns to this in her broadcast, and says 'Frankly, I do not myself feel at all like my Tudor forebear, who was blessed with neither husband nor children, who ruled as a despot and was never able to leave her native shores.'" If these (particularly the first two) are what defines a reign, then a year characterised by divorce and infidelity is indeed an annus horribilis."

After lunch, Starkey began walking her round the exhibition. "She didn't stop until she came to the portrait of Elizabeth that appears on the cover of my Elizabeth book - and then it was 'Philip!' Clop clop clop clop. 'Isn't this mine?' Which indeed it is. And that was basically her response. She knew her own possessions. She was like a housewife who'd been left them. She's looked after them, she's put in place much better arrangements for their care, but again - I suppose it's this absence of any kind of - to be blunt - serious education."

If there is indeed nothing but slavish duty behind the mask, then the monarchy must look outward for support - but here too is trouble. Ever since Henry VIII - Starkey's specialist subject; he is unapologetic about the mugging up he had to do to cover the rest of the 1,500 years - the Church of England has of course been the other face of monarchy. But "is it now time for the monarchy to throw over the Church of England?" he asks, thrillingly, in his programme. "After all, the sometime national church, despite the splendours of its architectural inheritance, is now in fact weak, divided, and fast shrinking into a mere sect. If religion still has strength in this country, it lies elsewhere, in evangelical Christianity and radical Islam, and neither is very promising material for royal ceremony."

To compound this, says Starkey, "the notion of public service has effectively been abandoned. Every political party now buys into business values, and into the notion that by definition business must run things more efficiently."

What's left, he argues, is a moral vacuum - and a position for Prince Charles to step into, if he so wishes. The accepted tone in which to talk about Charles is of course mockery so the way in which Starkey hails him as a possible saviour of the monarchy, and an active force for good in Britain, is initially a shock. "I was astonished at my own conclusion," he says, "completely astonished."

And it did not come, he insists, from a craven support for monarchy for its own sake. He admits to being seduced by narrative, biography, colour, but sternly says he is "a rational monarchist", who is perfectly capable of envisioning the abolition of the royal family. "We would have to engage in a really radical rethinking of our constitution ... which in some ways, I think, might be a rather good thing. We have this extraordinarily unbalanced constitution, in which we have an elected dictatorship of the prime minister."

Rather, Starkey's support for Charles comes from a sense of what he has achieved, and might yet achieve. Starkey has no tolerance for the further reaches of his experimentations - "when he starts whittering on about faith, you know, he makes me want to put my head in a bucket." But Charles is intellectually curious, he says; well read, a good speechmaker, an "excellent father", and willing to tear up the Windsor rulebook when it gets in the way: as it did when he wanted to marry Camilla Parker-Bowles. What really changed Starkey's attitude, however, were looking closely at Charles's interventions in education and heritage, his environmentalism, his campaigns on food - and especially the Prince's Trust.

The final flourish of Starkey's series is also a challenge. "Now there is a moral vacuum left by the sellout of the state to business interests, will King Charles step into the breach? ... Something new is required. Altruism, neighbourliness, the fruits of the spirit, are as important as ever. Who will speak up for them, if not the crown?" You may not agree with his conclusion, but it's a serious question to ask.

· Monarchy: The Windsors airs on 26 December at 8.30pm on Channel 4