As befits a man who has played a prescient spymaster for a decade, Peter Firth had sensed for some time that Spooks’ end was nigh. So when it was officially announced that the new series would be Spooks’ last, Firth says, pragmatically: “The swansong feeling has been there a couple of years and it’s better to go out on a high. Few shows run for ten years and if you learn from your mistakes and not make them the next year, but make some different ones and learn from them, then you build and build.”

And Spooks certainly built. From its launch in the spring after 9/11, when an interest in all things espionage made it the ultimate zeitgeist drama, it became one of the BBC’s most popular programmes, selling around the world and acting as a calling card for its producers Kudos (which was sold by its owners in 2007 for a reputed £35m).

As well as turning Matthew Macfadyen, Keeley Hawes and Rupert Penry Jones into household names, it attracted seasoned stars such as Jenny Agutter, Anna Chancellor, Hugh Laurie, Simon Russell Beale and, of course, Firth.

“I’ve avoided television series my whole career as I didn’t want to get stuck in something so-so, but I’m really proud of Spooks,” he says. “Every television show starts out with the best of intentions, but you never know what’s going to happen. By the end of the first series, which was only six episodes, it was clear we were onto something special.”

Aside from its judicious timing, what does Firth think that “special something” is?

“We’ve learnt that once you’ve got an audience on-side, they will forgive you a lot. We have holes in the plot sometimes that require a leap of faith and the audience make that because they know there will be a rewarding pay-off.”

Squeezed budgets also became an issue over the years. There are only six episodes in this final series “because of money” and stunts involving explosions, car crashes and helicopters have to be talked about rather than shown.

Still, the casting in Spooks “has always been brilliant” and it’s a drama that adheres to the winning principle of showing the exploits of a gang that we want to belong to. While he, and we, might divide Spooks into the Tom, Zoe and Danny years, and the Adam, Ros and Lucas era, Harry has remained constant throughout. And for all the high-octane drama, there has been a tender, unrequited love story at its heart – that of Harry and Ruth.

“The romance was never scripted,” admits Firth. “It was her and I flirting in takes, holding looks too long, lingering over lines. The writers picked up on that and it just grew from there. Nicola Walker, who plays Ruth, is a bit unpredictable – you never know what she’s going to do – but she’s an utter joy to play alongside.”

So will Harry and Ruth’s relationship be resolved? Firth, who is ridiculously sun-kissed from a holiday in Majorca, keeps shtum. Mostly. “This series is a six-act Greek tragedy with a profoundly tragic ending,” he says. “But there’s an optimistic note to tell you that the fight isn’t over.”

How typically cryptic.