And something good, too, about Pope Francis himself. “He seems to be a thoroughly decent human being. He chose to go on living as Pope in the Domus Sanctae Marthae [the hostel that houses cardinals away from the world during the successive rounds of voting] rather than go to the grand papal apartments. He lives a humble life and drives around in a little battered car.”

When researching Conclave, he was granted “remarkable” access to these private quarters. “They even stopped the lift outside his bedroom door and showed me it. Francis likes to eat with the nuns rather than in any grand way, and when he travels in the elevator and the doors open, he says ‘Come on in, I’m just another sinner’.”

But it is the forum of the conclave as a human, political and spiritual set-piece that has drawn in so many fans of the book and film. “One gets the impression of good people struggling with enormous contradictions.”

Most obviously, as Conclave chronicles, there are the deep divisions over Church teaching. “What it says on birth control, on divorce, on homosexuality, on the non-existent role of women, they are so against the current of the times that for me as an outsider it is a miracle that any institution can survive that.”

He has recently read, he says, a newspaper article arguing that “to an eerie degree the choice facing the next conclave will be the same sort of candidates as in my novel”. Does he think that is right?

“Well, there will be an African who is very anti-homosexual, a Filipino who is very liberal, an ultra-conservative, and an Italian diplomat. I say this merely because I look at the pattern and these factions have always been there.”

Harris draws the line at going on to suggest any real-life names. “I certainly wouldn’t say a liberal type; that would almost certainly be ruination.”

He is more comfortable discussing his own characters. “A surprising number of people have told me that their favourite in Conclave is Cardinal Tedesco [the ultra-conservative, played by Sergio Castellitto, who believes there are too many Muslims in Europe] because he stands for something and he argues it forcefully.”

For Harris, it suggests that the Catholic Church, at its core, is considerably less liberal than its current leader. “They don’t want a lot of wishy-washy, well-on-the-one-hand and then on-the-other. That is why the Church of England is now in a terrible state because it blows with the wind.”

Another aspect of conclaves is that, despite being shut off from the world, global events still have an impact. When, in 1978, the Polish John Paul II emerged as the first non-Italian pontiff in centuries, he played a prominent role in the fall of the Iron Curtain.

Might, then, the age of Trump, Putin and the rise of the hard-Right in European elections prompt an appetite for a more traditionalist pope? “I’m getting old now,” Harris says, “and I have come to the great revelation of my life, that things aren’t rational. Human beings make emotional choices.”

He points, by way of example, to the impact of the internet. “It should be a great tool of the Enlightenment, of reason. Instead, it has unleashed incredible forces of superstition, conspiracy theories and so forth.”

His words are a reminder that, as a historian, his novels seek out wider lessons from the past for today’s world and so bring us neatly to Precipice. Set in the summer of 1914, on the brink of the First World War, it revisits the love affair between prime minister Herbert Asquith, and Venetia Stanley, half his age, a passion so obsessive that Asquith broke the rules to share with her top-secret documents that Harris suggests impacted on the course of political history.