AT the London launch of his most recent spy book, Charles Cumming was chatting to actor Helen McCrory.

McCrory, who is married to Damian Lewis (who was also present at the launch, chatting with former intelligence operatives) had a role in the James Bond film Skyfall as a government minister chairing a Board of Inquiry into security matters.

Now, over a glass of wine, Cumming made her a playful offer: how would she like to be the head of MI6? McCrory jumped at the chance. We can only hope that Colin Firth agrees that this would be an astute piece of casting.

To explain: Firth's production company, Raindog, has bought the film rights to Cumming's two-thirds complete MI6 trilogy. Shooting is expected to begin next year.

Firth's interest is just the latest milestone in the career of Cumming, who has, the trilogy aside, written five well-received espionage novels.

His 2001 debut, A Spy by Nature, was followed by The Hidden Man, The Spanish Game, Typhoon, and The Trinity Six, a potent update of the Cambridge Spy Ring that included Kim Philby, Guy Burgess, Donald MacLean and Anthony Blunt.

A Foreign Country, the first part of the trilogy, took the CWA Ian Fleming Steel Dagger. And some critics think Cumming is the best spy novelist we have.

He is Scottish by virtue of the fact that his parents moved here in the mid-sixties. "My dad is a Scot, my mother a Scot via Australia," he says in an Edinburgh hotel. "He had a wine business in Ayr for many years, and my sister and I were born there, in a place called Thornyflat.

"I drove past there the other day. It's now a building site. I don't know what happened to the hospital I was born in, but it's gone."

Aged eight, Cumming attended a boarding school in England. Later came Eton, then Edinburgh University, where he won a first-class degree in English literature.

In 1995 he was sounded out about joining MI6. The approach came to nothing, and though he wasn't to know it then, the experience would eventually come in useful.

"I wasn't disappointed, no, that nothing came of it," he says. "I didn't really want the job, and told them so in a roundabout way. I've never known why it didn't happen. It might simply be that they sensed I was an artistic person, a solitary person, really, and that I possibly don't have the ruthlessness you need, in the sense of the ends justifying the means, of being able to square your conscience: you're going to go after somebody who doesn't necessarily deserve to be drawn into some Machiavellian scheme, but that by doing so, you will save lives.

"I don't really have that sliver of ice in my heart," he adds, paraphrasing Graham Greene, who unlike him was in MI6.

Cumming re-located to Montreal in 1996, where he began work on A Spy By Nature, which was partly based on his experience with the Secret Intelligence Service.

"It was called Peripheral Vision as I was writing it, which took me all of three years. Peripheral Vision," he laughs. "The biography of an optician ...

"I didn't think of it as a spy novel. I didn't have such novels in my DNA. I'd had this experience with MI6 and partly because of that had read a lot of John le Carre, but I was not in any sense an aficionado of the genre.

"What I was trying to do was to mould the tone of le Carre's The Spy Who Came In from the Cold with, believe it or not, the chatty style of Nick Hornby."

Penguin bought the book but hated the title. The new one was suggested by his editor. Cumming was now a 'spy novelist'.

"But it was just a very lucky sequence of events," he insists. "The themes of the genre suit elements of my personality, temperament and interests, in the sense of decent people behaving malignly, or the idea that there is good and bad in all of us. That stuff continues to resonate with me.

"But it wasn't a conscious decision to go down the le Carre or [Len] Deighton route: it just seemed a natural fit for me."

His most recent book, A Colder War, has a disgraced MI6 agent, Thomas Kell, on the trail of a traitor who is jeopardising the West's intelligence operations. The title deliberately evokes the Cold War between East and West (topically so, in view of some fears voiced in the West as a consequence of Vladimir Putin's recent behaviour), while the book speaks to the British agencies' concern lest there is another Philby in their midst.

"Philby defected in 1963, the same year as The Spy Who Came in from the Cold was published," says Cumming. "To this day, if you mention Philby to serving M.I.6 or M.I.5 officers, the colour drains from their face. It's a paranoid nightmare that they all live with: that the person they share lunch, and secrets, with could be a traitor.

"I've tried to find out how people feel about Edward Snowden [the US contractor who leaked thousands of documents about America's surveillance activities]. They seem calmer.

"I think that's probably because he's American, and most of the secrets he gave out were American, but also because he doesn't know the identities of agents.

"That's the other thing they dread: agents getting blown. Because the whole business rests on trust. S.I.S. needs to have a reputation as an organisation that looks after the people it recruits."

Cumming flatly rejects the notion of Snowden as a champion of free speech, viewing him as a cross between a hypocrite and an anarchist.

To paraphrase his argument, Cumming believes the intelligence agencies would be guilty of an "outrageous dereliction of duty" if they didn't have the ability to listen to our mobile-phone conversations or read our text messages.

Now, "the technology exists [for them] to do that, and they are, supposedly, there to protect us from bad people in the night ... GCHQ and the National Security Agency don't actually listen to your phone calls or read your texts: what they do is scoop up masses of information, and if you fall under suspicion, they have the ability to run a trace on you."

As for the third part of the trilogy, don't expect it to see it in the shops next weekend. He is lost in admiration "for those ambitious, writers who can write a book every year, but I tend to write a book every two years. I need time off to cogitate. Deadlines are important, but I don't think these books can be rushed."

He is, however, working on a screenplay for Trinity Six, while trying to develop a career as a screenwriter in Los Angeles.

By his own admission he thinks about intelligence and security matters more than most people. This approach is reflected in his books, which tend to be (to again borrow from Graham Greene) about 'the human factor' in espionage, focusing on character and relationships at the expense of, say, high-tech gadgets and gunplay.

"I couldn't do what Robert Ludlum or Ian Fleming did," says Cumming. There is an elegant pause. "Which is a shame, because I would have had a yacht by now."