Black Doves writer reveals "difficult" progress on season 2

But when her secret lover becomes the victim of a targeted assassination, she is determined to hunt down those responsible and make them pay.

Asked about the highly-anticipated second instalment at the Radio Times Covers Party, Barton told RadioTimes.com that he's been writing "every day".

He added: "It's really difficult. Yeah, it's difficult. I'm getting there. I just sit down every day and try and write it. We could be a little bit further along, but we'll be fine."

Speaking about the writing process behind the first season, which ended with quite the twist, Barton recalled: "I wrote it in a little writer's sprint on Boxing Day.

"In this case, it was the characters [that came first]. I liked the character of Helen, I had this idea about spy with a secret, and then I want to do a story about friendship, about how these two, a spy and a hitman, could be friends, and then all this sort of plot stuff came later."

As for the show's success, he added: "You always hope, but you never expect.

"It was the biggest thing I've ever done. Like, yeah, when we started seeing the viewing figures coming in, it was quite shocking, but nice."

Barton previously revealed that season 2 of Black Doves had already been given the green light before season 1 had even made its debut.

"It's never happened before," he said. "Most things I work on get cancelled almost instantly.

"So, I was all prepared to have a really nervous Christmas this year.

"I was on holiday, and in an Italian supermarket, when I got a call from the producers saying, 'Come home, you need to start writing, we're doing more.'"

Black Doves is available to stream on Netflix.

Check out more of our Drama coverage or visit our TV Guide and Streaming Guide to find out what's on. For more from the biggest stars in TV, listen to The Radio Times Podcast.