Sarah’s choice of smaller roles close to home was down to her determination to be around as much as possible while her daughter Nell, now eight, was young. 

But the title role in a meaty new ITV thriller, Bancroft, was too exciting an opportunity to pass up.

Sarah plays DCI Elizabeth Bancroft in the four-part drama – all shown this week – that promises to be a new twist on the cop genre. 

“I hadn’t played a big lead like that for quite a long time,” says Sarah, 49, who’s married to actor James Murray.

“But I remembered how lovely it is to be in all day, every day. You become a part of that crew, the set and the piece. You become that character and it’s much easier. I absolutely loved it.”

Bancroft is an ambitious, successful, maverick copper whose promotion to Detective Chief Superintendent looks assured – she’s been having great success trying to break the local criminal gang run by the Kamara family. But she’s got buried secrets that threaten to undo her. 

“I liked the fact that she had so many layers to her,” muses Sarah.

“There was so much going on for her at any one moment. She was incredibly complex and fiercely focused. It’s quite nice playing someone who isn’t always redeemable.”

Elizabeth’s secret is tied up with the unsolved murder, 27 years ago, of pregnant woman Laura Fraser. When a fresh-faced police detective, DS Katherine Stevens (played by Love, Nina star Faye Marsay), begins working on the cold case, Elizabeth realises that Katherine’s enquiries could cause her untold trouble. And what will she make of Katherine striking up a relationship with Elizabeth’s only child, Joe? 

Complicating matters is the fact that the dead woman was married to star barrister Tim Fraser, a man well known to Elizabeth. He’s played by Linus Roache, 53 – son of Coronation Street legend William Roache (Ken Barlow) – who has worked mostly in the US.

Linus has starred in Law & Order, Vikings and Homeland, but the fresh take on the crime genre drew him back to the UK. 

“How many shows have we got about criminal justice? Hundreds. But it’s always interesting when there’s a new take on it,” he says. 

“This was a role where you see the personal, the underbelly, the flip side of this character and you never get to see him in action as a professional.”

Tim, he explains, is in a failing second marriage but devoted to their daughter, Amber, when his world is rocked again by the re-opening of the investigation into his first wife’s stabbing. 

“Suddenly this case has come to life again and the past he has tried to move beyond is brought into the present,” says Linus.

“So Tim goes on this journey to discover what happened. We don’t quite know at first what Tim’s relationship is to Bancroft, but that emerges through the course of the drama.” 

Scenes taking us back to Laura’s murder in 1990 are acted out by Sarah and Linus rather than by younger actors – a special anti-ageing camera technique was used. All they had to do was restrict their facial movements in those scenes so that their faces could be more easily mapped by the computer technicians. 

“There were slight restrictions, but not anywhere near as much as I thought there would be,” says Sarah.

 “I saw tiny bits on screen when we were filming and it looks amazing. It doesn’t really look like you when you were young. It looks like a younger version of who you are now.”

The series begins in 1990, when young detective Bancroft is the first on the scene of the murder of Laura Fraser. We jump to 2017, when DCI Bancroft is working on cracking the Kamara gang and is considering applying for the post of Detective Chief Superintendent.

Meanwhile, DS Stevens moans to her boss, Superintendent Cliff Walker (Adrian Edmondson), that she wants a juicy case to work on and he hands her the unsolved Fraser case file, from which she notices important documents are missing. So begins the unravelling of the mystery of the murder and the secrets of Bancroft’s murky past. 

“Elizabeth is very ambitious and aiming to become a Detective Chief Superintendent, but there is something in her past she is covering up,” explains Sarah.

“And it is about to raise its ugly head and come knocking on the door again.”

Monday to Thursday, 9pm, ITV