Meet Bancroft's fiendish new villain for series two

But by the end of the episode, we find out the horrible truth: it was Annabel who killed her own family. And, with poor Joe as her pawn, she's planned out exactly how to get away with murder.

"She is an amazing manipulator, dressed up as this beautiful little girl who looks perfect," Parish tells RadioTimes.com. "But she’s just brilliant. And I think that’s a real challenge for Bancroft, especially because she’s going out with her son. It drives her to almost insanity by the end. But it’s quite interesting to see them fighting each other!"

Because of her son's connection to the family, Elizabeth is pulled off the case by her boss – much to her frustration. But she immediately realises Annabel is a fellow murderer.

"As soon as she sees her, she knows," Parish says. "You can recognise a killer if you’re a killer. There’s something about her. She just knows that something’s not quite right. And I think that Annabel knows. Do you know what I mean? They just see it in each other."

Writer Kate Brooke opted to bring in a new antagonist for Elizabeth now that DS Katherine Stevens is out of the picture – and this time she has created "a real contender" for her to reckon with.

"You’ve only hit the surface by the end of episode one," Brooke says. "But I hope, also, that I try to get the audience to understand why she’s got to that, and what happened to her in her childhood, and what her past is that makes her this arch manipulator... it continues to unveil a lot of stuff about Annabel, and there’s lots of twists and turns within that."

She adds: "I think that she’s a great little performer. She’s hiding a lot of stuff. She’s a player. She’s like a classic film noir character... you know, Double Indemnity or Body Heat – all these women and female characters who play everybody, and play them very successfully. Gradually, as the show goes on, you realise just how bad they are."

While series one focused on a cold case from decades ago, Brooke decided it would be "fun" to give audiences a new murder for series two.

"The cold case in season one was really about us finding out about what her deepest secrets are," she explains. "But season two is really more about: there’s this dysfunctional policewoman who’s really, really good at her job – and we throw a really different case at her, and throw a case which has got a personal element in it, and we see how that plays out."

But also, the screenwriter says, "I’ve always been interested in stories about people who kill their parents. I think it’s just really fascinating. A lot of season two is about mother-love, really, and how parents affect children."

When Elizabeth sees the picture of her son in the framed family photo at the crime scene, Parish says, "it blows her world apart."

"It’s him, and she hasn’t seen him for ages," the actress tells us. "And he is within this crime. So it’s a big head-f**k for her, basically.

"Also, it means that she can’t be on the case. She has to be taken off the case. So for her, that’s a nightmare. So she’s lost control of her son and the case, and you basically watch a woman spiral out of control over three episodes.

"She’s all about control. That’s what she is, you know? So to see her on the back-foot is really interesting."

Bancroft continues on 2nd and 3rd January on ITV