A few years ago, David Cornwell was walking with his son Nicholas on Hampstead Heath. The father was in his eighties, and he had something to ask his son, approaching his fifties.

“If I leave something unfinished,” he said as they looked out over London, “will you complete it for me?”

“I said yes,” recalls Nicholas, “mostly without thinking about it, because I wanted not to be in that conversation.”

Both Cornwells are somewhat better known under other names: Nicholas as Nick Harkaway, the author of five novels under that pen name, and two more as Aiden Truhen; and David Cornwell as one of the greatest proponents of spy fiction in history, John le Carré.

As Le Carré, he wrote 26 novels between 1961 and his death in 2020, many of them featuring George Smiley, and most drawing back the veil on the murky world of espionage, as well as providing us with its nomenclature – moles, honeytraps, scalphunters and lamplighters to name but a few.

His work has graced cinema and TV screens and there are few people who cannot name at least one of his books, be that The Spy Who Came In From The Cold, or Tinker Tailor Soldier Spy; Smiley’s People or The Night Manager.

George Smiley and Le Carré’s other recurring characters are the shadowy antithesis to the hi-tech, glamorous world of James Bond. Smiley’s people work in the grubby side-streets, in rooms lit by bare bulbs, under cover of darkness.

Le Carré’s characters have been brought to life on screen by Ralph Fiennes and Sean Connery (well past his 007 years in the 1990 adaptation of The Russia House), while Gary Oldman and, perhaps most famously, Alec Guinness, have both played Smiley himself.

The subtext of that conversation Harkaway didn’t really want to have with his dad on Hampstead Heath eventually came to pass when Le Carré died in December 2020, aged 89, following a fall at his home.

It was a rough couple of years for the Cornwell family; Le Carré’s wife and Harkaway’s mother, Jane, died just two months after her husband. Timothy, one of Le Carré’s four sons (Simon and Stephen, along with Timothy, were from Le Carré’s first marriage) died aged just 59 in 2022. Le Carré’s half-sister, the Cornwell children’s Aunt Charlotte, also died shortly after.

“It was a bad time to be a Cornwell,” says Harkaway, 52, the son of Le Carré and his second wife, book editor Jane Eustace. “You felt like you were in the sights, a little bit.”

But following his father’s death, Harkaway was called upon to make good that promise on Hampstead Heath. For John le Carré had indeed left an unfinished novel. Silverview was eventually published posthumously in October 2021, completed by Harkaway, though he claims in the author’s notes that his contribution was “more like retouching a painting” than anything else.

“I did a tiny polish on it to get it from typescript stage to publication,” he says modestly. “And I was like, ‘OK, I’ve done it now. He extracted a promise from me and this was the easy version of fulfilling that contract.’ It was fun to do, and it was also touching to work on that.”

However, just like promises made and deals brokered in the shadowy world of Le Carré’s novels, things were not going to be quite that simple. Hence, this week, Karla’s Choice was published. A new novel by Nick Harkaway, but in the world created by John le Carré. And not just that, but starring his most famous creation: George Smiley.

Karla’s Choice is set in the decade between The Spy Who Came In From The Cold and Tinker Tailor Soldier Spy – the ten years Le Carré himself never chronicled. It is the spring of 1963. Smiley has quit spy network “The Circus”, hoping for a life of peace.

But, of course, this retirement doesn’t last long, and Smiley is reeled in for one last job, investigating a missing Hungarian emigré linked to a Russian hitman. And it’s not long before Smiley realises that walking away from the job is a lot easier said than done.

Harkaway really has done his father proud. The genteel shabbines of the spy headquarters if perfect and familiar characters such as Toby Esterhase and Bill Haydon appear to help the plot along. Reviews, unsurprisingly, have been largely ecstatic. Despite the promise made to his father before he died, and although Harkaway is a critically acclaimed author in his own right, the two writers worked in very different worlds.

Though not without intrigue and thrills, Harkaway’s novels, such as The Gone-Away World, Gnomon and Titanium Noir, are high-concept, science fiction-tinged books of big ideas. Still, when you’re the son of John le Carré, perhaps spy writing is somewhat in the blood.

Harkaway says: “This is something I didn’t realise until I started talking about it to people in the last few weeks. I was born in 1972 and my dad used to wake up in the mornings at about six, sometimes earlier, and he’d potter around and start working and then he’d read to my mum what he’d written.

“I would hear him reading his work to her in their bedroom down the corridor, the two of them talking about it. And that’s how I learned to talk. Literally, part of the process of me acquiring language was listening to mum and dad reading the Smiley novels out loud as they were being written.

“Whatever part of your head learns and absorbs language and determines how you use it came from that, it was one of the first layers of language I ever heard.” Harkaway had read his father’s books, of course, many times, as well as listening to audio adaptations and watching the TV and movie versions.

He was well-versed in, what he calls with a wry smile, “the Smileyverse”.

Still, when it was suggested that he write a new George Smiley novel, it still came as something of a surprise to him. The sons, and Harkaway’s wife Claire, essentially run “John le Carré Ltd”, the literary estate of the writer. Le Carré is a brand, and to maintain a brand, you have to keep visibility of the product and its associated spin-offs.

Following Cornwell’s death, a letter was discovered from the writer asking his family to ensure the books continued to be read and to reach new audiences. “You need to keep the books in the public eye, keep people reading them, you need TV shows and adaptations,” says Harkaway today. “And in this case, one of the things you can do, which is a lot less expensive than a movie or TV series, is a new book. And if the author of those books is dead, then you’re going to have to get someone to do it for you.”

He says the family tossed ideas around, and Harkaway was thinking: “We’ll get someone amazing with a huge kind of literary canon, you know, and we’ll ask them to write something that is in the Smiley universe, but also their own thing. And that’ll be a great thing to do.”

The question was, who could carry it off well enough to satisfy fans of a 60-year writing legacy?

It was then that Harkaway’s film producer brother Simon pointed out to him: “You know, there’s a pretty clear logic here that it should be you.”

Harkaway was reticent at first, admitting: “It’s a massive undertaking, it’s one of the most extraordinary fictional universes we have in the 20th and early 21st centuries. Arguably, no one can continue it. But then… it’s a massive opportunity, an extraordinary chance to play in this remarkable world.”

He pauses, then adds: “You know, I never really talked about writing with my dad. I never had what you might call a literary apprenticeship with him. But doing this feels a bit like exactly that. And I knew people would think I couldn’t do this, which immediately makes me want to prove I can.”

And he did. Mick Herron, author of Slow Horses, calls Karla’s Choice a “note-perfect tribute to Le Carré”. Ian Rankin says: “Le Carré’s legacy is in good hands – Smiley is back with a vengeance!” And Richard Osman adds the book “reads like a lost Le Carré. Smiley is back at The Circus in the safest of hands”.

The big question, though, is what would the man himself think?

“You can’t know the answer to that,” says Harkaway. “But I know what he would have asked me. He would have said, ‘Do you know why you’re doing this?’ Not just in terms of pure motivation, but creatively. And I think I do.

“I have this recurring thing when I’m writing where I feel like I should turn around and he’s sitting there, sort of grinning if it’s going right or shaking his fingers, like, ‘You’re screwing this up’, in an Obi-Wan Kenobi sort of way.

“But I also get the sense that my head is doing the same thing as his did in those moments, and it’s not like talking to him or being with him, but a little like being like him in that specific way. And therefore understanding him, though I don’t have a great desire to turn into him. But somehow this puts us closer together.”

Having now taken up the mantle of his father’s most famous legacy, it has to be asked… is Harkaway going into the other family business? Le Carré worked for both MI5 and MI6 up until 1964. Has Harkaway also been tapped up to be a spy?

He laughs uproariously. “You know, I’ve always been faintly offended that no one’s ever asked me. And at the same time, I can completely see why you wouldn’t want someone like me doing that job!”

Karla’s Choice by Nick Harkaway (Viking, £22) is out now. Visit expressbookshop.com or call Express Bookshop on 020 3176 3832. Free UK P&P on orders over £25