“I’m Catherine, by the way. I’m 47, I’m divorced, I live with my sister, who’s a recovering heroin addict. I have two grown-up children. One dead and one who doesn’t speak to me. And I have a grandson.”

This is the introduction to Catherine Cawood, a straight-talking police sergeant working in rural west Yorkshire and the focus of BBC1’s new six-part drama, Happy Valley.

Played by Sarah Lancashire, Catherine’s life is troubled by work and personal problems. Her daughter, Becky, is dead and the man she holds responsible, Tommy Lee Royce (James Norton), has been released from prison and may have returned to the area.

“From about halfway through the series, it becomes clear that the story isn’t really about Catherine’s job as a police officer, but about her trying to come to peace with herself,” says Sarah, 49. “It’s about a woman who is very damaged by the loss of her daughter.”

Happy Valley is written by Sally Wainwright, who also created Scott & Bailey, At Home With The Braithwaites and Sarah Lancashire’s last TV project, Last Tango In Halifax.

“Sally’s scripts are like transcripts of people’s lives and they breathe immediately,” Sarah told the Daily Express. “You don’t have to think about it too much. In fact, it’s often best to let her writing do it for you. She’s just top of her game, really. Happy Valley is an extraordinary piece.”

Sally says she wrote Happy Valley and the part of Catherine with Sarah in mind.

“When we started filming Last Tango, like so many other people, I was completely blown away by Sarah’s performance,” Sally says. “As a writer you feed off performance – if you see a great performance it makes you want to write more and more. When Ben Stephenson [BBC Controller, Drama Commissioning] asked me what I wanted to do next, one of the factors that made me want to create Happy Valley was that I wanted to write something else for Sarah.”

But Happy Valley is not just the story of a woman’s grief. Catherine has a job to do, too and has to act on her suspicions when a nervous accountant, Kevin Weatherill (Steve Pemberton), arrives to report a crime, but loses his nerve and leaves before giving details.

The crime Kevin intended to report is of his own making: a plot to kidnap his boss’s daughter. However, after asking local drug baron Ashley Cowgill (Joe Armstrong) for help with the abduction, events spiral out of control and Kevin is soon regretting the whole idea.

Sarah says playing a sergeant in Happy Valley, which also stars Siobhan Finneran, George Costigan and Adam Long, has given her a new appreciation of the police.

“I went out with the police in Calderdale. It was fantastically useful,” she says. “Our police adviser was always on set to help with the procedural stuff – she had a very keen eye for accuracy. I really did need her there all the time, just for tiny things like putting on handcuffs or, in one scene, delivering bad news. She told me, ‘Look it’s OK to cry and hug them’.

“One of the most interesting things I learned was how undermanned the police are and that their life can change within five minutes. We had this incident where we were called out to a house where the bailiffs had turned up and needed to take some belongings away. It turned into the filthiest fight and that’s when I stayed in the van. The whole day changed. It’s interesting, but it’s not for me.”

Happy Valley, BBC1, Tuesday, April 29