The conventional wisdom re William Hague is that success happened ten years too early, that he became Tory leader too young and at a time when the Conservatives could not possibly win an election. The regret of the Conservatives’ youngest leader since William Pitt is that he entered Parliament 200 years too late.

“I really wish that I’d been born in the 18th century. I’d have been there with Pitt and Wilberforce,” he says, name-checking the subjects of the two bestselling biographies – of William Pitt the Younger and the antislave trade campaigner – he has written since stepping down from the leadership in 2001.

He sits in one of those corner offices in Portcullis House that grant former leaders a view, at least, of the ebb and flow of power in Parliament Square. At 45, he looks happy, fit and slim in his blue wool suit. He is easily 6ft tall but his head, although what remains of its hair is ruthlessly cropped, remains a mite too large for his body. The late Tony Banks jibed that the Conservatives had elected a foetus.

His unsuitability for the television age is only one reason that Hague the Now Not So Young hankers for the 1800s. “The difference is that you could still affect the result by making a speech. That is very rare now. I think in the debate on the Iraq war in March 2003, the speeches made a difference, particularly the speech of the Prime Minister. The result was a foregone conclusion but not the size of the majority.”

And didn’t he make a rather effective contribution himself? “I’m not sure if that made any difference, but Tony Blair thanked me for it, and I said, ‘Don't expect me to ever do that again’. But such occasions are very rare now, whereas in the days of William Wilberforce . . .”

How Hague harks back to the era of Pitt and Wilberforce! To some of us, however, he will forever be that reedy-voiced, floppy-haired schoolboy who wowed the 1976 Conservative conference by demanding the state’s frontiers be rolled back and replaced by (before the noun had even been invented) Thatcherism. I ask if he was sickened when Gordon Brown invited Baroness Thatcher for tea the other week. He wasn’t. “I think one of the first things any Prime Minister should do is invite Lady Thatcher. Hopefully he’ll learn a few things.”

Does he still see her? “Yes, absolutely.” And is she as frail and forgetful as Rob Wilson, a Tory education spokesman, suggested when he said that Brown had manipulated her? “Well, if she was, I wouldn’t say so to a newspaper. On most occasions that I see her she is on far finer form than you would ever think from accounts that I read afterwards. I sat with her at the commemoration of the Falklands Campaign a few weeks ago and she was very strong on her recollection of all of those events and the decisions that were taken and why she had taken them.” And so to the other two Williams. He has affinities with both. Pitt, Prime Minister at 24, makes Hague’s own precocity look positively sluggardly, but the comparison is there. William Wilberforce, like Hague, was a Yorkshireman. Pitt and Wilberforce were opposites, however. I say that they seem like two sides of the complete politician Hague might aspire to be.

“They are two sides! Really, I regard these as sister books. Pitt is the ultimate career politician: Prime Minister at 24, dead at 46, having worked and worried and drunk himself to death. Wilberforce, same age, enters Parliament at the same time, great friend of Pitt, but becomes the ultimate noncareer politician who exercises more influence than most prime ministers while never actually holding office himself.”

For his first decades as a politician, Hague was the careerist whose rise was checked only by the electorate. If you want an insider to spell-out his shortcomings as a party leader, look no farther than the man himself. “I’m very interested in foreign affairs and some other areas of policy, but I probably did not have the all-round interest that a party leader needs, and not necessarily the patience to spend every day dealing with everybody’s little complaint that the party leader needs to have. So I think I have certain flaws. Those probably became apparent when I was a party leader! And I’ve no intention of ever doing it again.”

His political judgment was awry too. In his first speech as leader he pledged to double the party’s membership and ensure that half of the new recruits were under 30. “This turned out to be completely unrealistic,” he says, blaming in part a general social trend against joining anything. Worse was the political zigzag his four years’ leadership described. He began by marching the party towards the middle ground, found he was going nowhere in the polls and, realising he needed to find some votes, tickled up the Tory heartland with a right-wing antiEurope election manifesto. “Well, that’s a bit of a caricature. The issues I raised were things that went well beyond the heartland. There were Labour voters who worried about crime, worried about the abuse of the asylum system and so on. However, those issues were not sufficient to get them to switch their votes.”

And Labour won a second landslide? “Yes, and it is clear after the last three elections that I think what David Cameron is doing is right, that we have to recapture the centre ground. And he’s set about it very energetically, very effectively.”

Hague, who serves as Cameron’s Shadow Foreign Secretary, is always loyal to his leaders, but his enthusiasm for Cameron, despite the dismal recent polls, seems genuine.

“He is very good. He is, all round, the most accomplished politician of all of us who’ve led the party in recent times.” Since Thatcher? “I think since Thatcher. But he’s also got real tenacity, which I think people have seen in recent weeks. The media has had a big downer on him this summer, but that hasn’t ruffled him. He has got the steel to come out the other side. I have huge respect for the man, actually. And I wouldn’t be doing it otherwise. You know, I don’t need to do it. I haven’t come back for any other reason than to try to win the next election.”

Hague is an older and wiser fellow these days, but the younger man fascinated me. His rise seemed effortless, from South Yorkshire comprehensive to Oxford to a business consultancy and then the super-safe Tory seat of Richmond, West Yorkshire. By 1995 and his early thirties, he was in the Cabinet as Welsh Secretary. His private life seemed equally jammy, for it was in that job that he met his future wife, Ffion Jenkins. Asked around the time of the 2001 election what were his worst times, I recall him mentioning the death of a grandparent. He looked quite untouched by life’s normal setbacks. “I think that’s a fair point. I’m really a most extraordinarily lucky person. And so it was an unusual setback for me to lose the 2001 election, and, of course, I had to resign as leader, but it was the most liberating thing that ever happened.”

Still, it must have been a blow? “Yes, definitely. But not for long. The real shock was how rapidly I got on with the rest of my life and didn’t look back. I discovered that I could write books and play music and could travel the world and my income soared and my spare time soared.”

He said at the time that he was hoping to start a family. Any signs of children? “Oh, that wouldn’t be something I would tell the press about.” For the only time, he shuts down an avenue of inquiry.

He says that as he gets older he regresses further into youth, loving music and sport. Yet his sympathetic biography of Wilberforce, whose private Bill abolishing the slave trade became law 200 years ago last February, suggests that its author still dreams of changing the world.

He would get his chance, of course, as Foreign Secretary in a Cameron government, but I wonder if Wilberforce’s example might not provide an alternative route to greatness. Much more than Hague, Wilberforce was not prime-ministerial material. Indeed, his poor eyesight and ulcerative colitis made him literally unfit for office.

The biographer has none of his hero’s congenital disadvantages but has shown some of his courage and independence of thought. On a visit-to America last year, he condemned Guantanamo before the Government did. That summer he called the Israeli attack on Lebanon “disproportionate”. If he was looking for a cause of Wilberforcian merit and urgency to champion in the second half of his career there is an obvious one to hand.

“The direct parallel is the modern slave trade. There are more people in slavery of some kind today than were ever carried across the Atlantic in the 18th century. And slavery is openly practised in parts of Africa. In Mauritania they made it illegal only last month. There’s a vast trade in sexual slavery, including to this country.” His biography, he says, has helped to raise money for the Stop the Traffik coalition and he makes speeches on the subject. Yet, he says, people are not listening.

“In a way you can say, well, we need a Wilberforce figure.” Why not him? “I will never pretend to be a Wilberforce. I don’t think that there is any comparable figure to him. I think that it is extraordinarily rare to forego political power to have long-term political influence instead. I certainly haven’t made that choice.” Because it requires too much suppression of ego? “Partly, yes. But it’s even truer today than it was then that to effect change or improvement you have to be in government. In the days of party majorities in the House of Commons, it would be harder to be a Wilberforce, even if someone had the moral and intellectual equipment to do so.”

But we can both think of individuals, aligned to parties and yet, like Wilberforce, independent in spirit, who have campaigned on single issues and won. Within Parliament, David Steel’s Bill legalised abortion and Sydney Silverman’s campaigning led to the abolition of capital punishment. Outside it, Bob Geldof shamed the West into writing off much Third World debt.

Maybe Hague is too much the parliamentarian, too much the conference darling, too much the disappointed idealist, too much the Pitt, to follow the example of Wilberforce. Or maybe, should his party be granted one more election defeat, he will eventually decide that he has been born neither too early nor too late, that, rather, the burden of his ambition has been lifted at just the right age. That there is time – and a way – that he might yet achieve something quite extraordinary.

William Wilberforce The Life of the Great Anti-Slave Trade Campaigner, HarperPress, £25. William Hague addresses the Times Cheltenham Literature Festival on October 7. To book, call 01242 227979 or visit www.cheltenhamfestivals.com.