Former MI5 trainer Ava Glass writes pacy spy thriller The Chase

So she was glad when a new colleague befriended her in the kitchen while making coffee.

"She worked in the legal department, she was about my age, perfectly ordinary in every way," says the Finsbury Park author.

"For three weeks she became my office buddy; hanging out, having coffees and lunch - she was really interested in my background. And then she was gone."

The penny dropped that this was her final security check before getting access to sensitive information.

"She was the first spy I ever met and I didn't know she was a spy. She was so plausible and normal, I hadn't the slightest inkling. I had such respect for her. As a journalist, I know you catch more flies with honey than vinegar."

That unobtrusive spook inspired Glass' heroine Emma Makepeace, the 28-year-old agent in race-against the clock thriller The Chase. Emma has 12 hours to deliver her asset into protective custody while being pursued across London by Russian intelligence. The catch is she must do it in one of the most surveillance-intense cities in the world.

The Chase is out now in paperback published by Penguin (Image: Courtesy of Penguin)

"I wouldn't have written the novel if I hadn't worked with spies," says the former crime reporter.

"When I joined, it definitely wasn't my world and I always felt like an outsider, but I met so many people I found absolutely fascinating."

The Texan spent five years in counter terrorism communication, trying to get MI5 and MI6 agents to be more open with the public via social media.

"I would give long presentations about Facebook, show them videos, and at the end they would say 'absolutely not'. They showed me some of what they do so I could understand. Much of their job lies in anonymity, so it was how to bridge the gap between their need for privacy and the need for people to know about the work they do.

They were so smart, some spent their time trying to think like a terrorist to see the vulnerability, others were analysing data to find hidden information. There were many attempts while I was there, but not a single major attack. That's not chance, that's hard work."

After leaving British intelligence, Glass' thoughts kept turning to that first spy who befriended her.

"We think of intelligence as very male, but women make such good spies. Maybe it's prejudice that it didn't occur to me such an indistinctive woman could be a spy, but it makes them incredibly successful. They live this life of tremendous secrecy, if you lived next door to one you would simply never know, but it also gets in their heads."

Emma is tasked with protecting the London-raised son of Russian dissidents who is a revenge target. Yet she herself is seeking revenge for her own father - a Russian government official, who shared secrets with the British after the Cold War, and was executed while she was in womb.

"It's become clear in the last decade that the Cold War never ended. We fooled ourselves into thinking we are good friends, the past is the past, but Russia never forgives or forgets, and Emma doesn't either," says Glass.

The Chase highlights London's surveillance network, including facial recognition cameras which switch angle to follow subjects.

"When I first moved to London I noticed them and felt watched, but now I don't even see them. Emma knows there are dark spots where you can move around unseen; parks, canals, an underground river, but eventually you have to come up, and the second you are on the street you are done. She knows it's a matter of time, and she has to have a plan for what will happen when that occurs."

Even before it had a publisher The Chase was pursued by TV production companies. John Le Carre's sons Stephen and Simon Cornwell's, Inkfactory, the company behind The Night Manager and The Little Drummer Girl won the option.

"I'm glad, they are the best in the business."

If she had a say, locations would include "some of the amazing old government buildings I have been in that don't feel like a government office, with sweeping marble staircases, and art on the ceiling."

And there could be a sequel because Glass is already on to her second book, The Traitor, which features Emma trying to solve the murder of a fellow agent.