Nick Harkaway was a successful novelist in his own right when his brothers asked him to continue their late father’s spy series. Could he pull it off?

• Exclusive extract from Karla’s Choice: A John le Carré Novel by Nick Harkaway

It takes a few anxious minutes for me to ring on Nick Harkaway’s north London doorbell, on the grounds that neither the taxi driver nor I is convinced it’s a private residence; its blankly businesslike doorway and proximity to a pub and an undertaker would lead a passerby to think of a discreet accountancy firm or therapist’s office. Or, perhaps, a safe house. But I double check the address and, seconds later, there is the proprietor, ushering me in.

Two hours later, I’ve discovered that there’s more to the history of the Hungarian tobacco industry than meets the eye, that Harkaway tracks the shipping traffic in the Danube corridor via an app and that, despite not being a horsey person, he has a surprisingly detailed knowledge of how the late Queen’s horses were trained. And I’ve also got a little nearer to understanding how he came to take on one of the most daunting literary challenges imaginable: continuing the legacy of his father, John le Carré.