Human bones, a barrel, the bottom of a reservoir. As far as bleak British crime dramas go, this seems like a textbook set-up.

Thank goodness, then, that the uniformed officer trudging through the mud to identify the dismembered body is Sergeant Catherine Cawood, played by the endlessly watchable Sarah Lancashire and a sign that what we’re about to embark on could not be further from a formulaic police procedural.

It’s been a long, near-seven years’ wait for Happy Valley’s third series. Writer Sally Wainwright wastes no time revealing that Sergeant Cawood hasn’t softened with age. “Sarah really owns Catherine. She’s just brought so much to that part. She’s so vulnerable and she’s tough. It’s just a fabulous balance that she’s found with this character,” Wainwright says.

In the rollicking opening sequence, Cawood is met with a series of withering remarks from the attending senior detectives as she shares her observations on the uncovered reservoir bones. “We’ll let the pathologist decide that, shall we,” smirks Vincent Franklin’s Detective Superintendent Andy Shepherd.

What follows next is a verbal evisceration so composed, rhythmic and characteristically Catherine that it could pass as poetry. Within minutes, Cawood has identified the victim as a local lad she’s had run-ins with years before. The detectives are rendered speechless. Viewers are left slack-jawed in admiration. “Anyway, I’ll leave it with you,” she says as she turns away. “Twats.”

Mixing strength with radical vulnerability, Lancashire’s humane performance in the first two series of Happy Valley left us spellbound, and even provided us with laughs amid the show’s grimly realistic portrayal of crimes that otherwise might have been too brutal to bear.

Despite the prolonged hiatus, few will forget what made Happy Valley a masterpiece when it first aired in 2014. Against the backdrop of a drug-troubled town in West Yorkshire, the series followed a police officer’s attempts to hold her grief-stricken family together after the man she holds responsible for her daughter’s suicide, the rapist Tommy Lee Royce (played by James Norton), is released from prison.

With a dead child, an ex-husband, another child refusing to speak to her, Catherine navigated the most gruelling situations that middle-age could throw at her, all the while raising a grandson with her ex-addict sister.

“[Catherine] is very vulnerable and she’s very flawed, and she has a temper, and she has a passion that sometimes takes her in the wrong direction. She’s extraordinarily complex,” says Nicola Shindler, executive producer of the first two series of Happy Valley, the second of which aired in 2016.

“What Sally is brilliant at is seeing the heroic in the ordinary,” she adds. “It’s a job. You get up and you go to work. It just so happens that the work you’re doing is sometimes life and death and really important. Sometimes though, it’s really mundane.… It was that element that made it feel very real.”

‘Everything’s going to pot. How can we cope without people like her?’

Series 2 concluded with Catherine fearing that nature might overcome nurture in the case of her grandson, Ryan (Rhys Connah), whose violent outbursts at primary school appear to be a sign of his innate capacity for evil, inherited from his father, Tommy Lee Royce.

With her grandson now 16, Catherine must reckon with the fact that she can no longer control whether Ryan wants his father to be part of his life. “One of the reasons we wanted to wait a while to do the third series was so that Ryan was old enough to be making his own decisions about things, to be more mentally able and astute and inquisitive about his own life in a way that he now has some agency,” says Wainwright.

Catherine’s sister, Clare, is often left to pick up the pieces. A former heroin addict, Clare’s relationship with her older sister is one wrought with pain, dependency and past trauma. “That relationship gets tested more than ever. It’s a massive part of the story,” Wainwright discloses. “They do have a really distressing falling out.”

At work, Catherine and her team are still trying to curb the Valley’s chronic drugs problem. The new series will see the force crack down on the illegal supply of prescription drugs, which are proving to be more prevalent and more destructive than Class As.

“Catherine always feels like she’s mopping things up and the bigger problem never gets dealt with. But, in this series, we get a bit closer to the problem and meet some of the bigger players who are supplying drugs in the area,” Wainwright reveals.

After 30 years in the force, Catherine is heading towards retirement and a life off duty, in her own words: “Code 11 – job done”. Having long found work an escape from her chaotic home life, how will she cope without it? “Oh, I think she’s quite happy,” says Wainwright.

Of course, the real question isn’t whether Catherine will cope without work, but whether we will cope in her absence. “The biggest thing for me was worrying about, if Catherine retires, will everything break down?” Wainwright explains.

“A friend of mine said to me the other day, ‘You do realise we’re watching the breakdown of civil society at the minute, don’t you’, and I was like, ‘Oh my God, yes, we are’. It does feel like Catherine retiring is a reflection of that,” she continues. “Everything’s going to pot and how can we cope without people like her?”

‘I do worry that I am quite soft on the police in shows like Happy Valley’

Despite her occasional missteps, at work Catherine is driven by a strong moral code. With the police mired in scandals, public confidence in the service has plummeted since Happy Valley last aired.

As the horrifying reports of misogyny, racism and sexual impropriety within the force appear to be increasingly symptomatic of a rot at the top, rather than the work of a few bad apples, Wainwright found herself grappling with how her presentation of the police might be received.

“I do worry that I am quite soft on the police in shows like Happy Valley,” says Wainwright. “There’s a problem with TV in general, where we like cop shows, and we tend to present cops as the heroes –and, obviously, I do with Catherine. Catherine is meant to be a really good police officer.

“And then we hear all these terrible reports of what’s going on in the Met and other constabularies.…

“Some of the stuff that’s been in the press recently about people who are inappropriately recruited into the police is mind-blowing.”

Talking to Happy Valley’s police consultants Lisa Farrand and Janet Hudson, Wainwright discovered that misogyny persists in the force. “They both said to me, you’d like to think things have got better, but they haven’t,” she says. Still, through her discussions with both women, she was reassured that goodness prevails. “I see that [Farrand and Hudson] care enough to have seen out their careers in the police. You know that they do care because of the way they talk about the job; they have done extraordinary things with true responsibility,” she says.

As Catherine prepares to hang up her hat, it’s been confirmed that the third series will be Happy Valley’s final. While the show was initially written as a one-off, after Wainwright decided to return to Catherine and her story with series 2, the writer always had a clear idea of how the drama would end.

“I wanted there to be some sort of big final confrontation between Tommy and Catherine. That’s always been the plan to conclude the story, to find a way to give some kind of resolution, one way or the other,” she teases. “We couldn’t have avoided it. I don’t think the drama would have been satisfying if they didn’t have a big explosive showdown.”

Happy Valley’s third and final series starts on BBC One and iPlayer on New Year’s Day at 9:00pm; series 1 and 2 are on iPlayer now.