“I don’t do this to find dead bodies.” Det Supt Steve Fulcher turns to his wife, two days into the search for missing Swindon girl Sian O’Callaghan.

“I’ve got to believe I can save her.”

Millions were expected to tune in last night to the first episode of ITV’s A Confession. The six-part primetime drama had been advertised on national newspapers and extensively trailed.

Last night, it opened with the first hours of 22-year-old Sian’s disappearance.

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It was Saturday, March 11, 2011. The office administrator had been enjoying a night out in Old Town with friends. CCTV recorded her leaving Suju nightclub and getting into a dark estate car that would later be identified as killer cabbie Christopher Halliwell’s vehicle.

For those who tuned in to last night’s blockbuster, there was no mention of Halliwell. Instead, directors left the episode on a cliffhanger. Mr Fulcher, played by Hobbit star Martin Freeman, is told of the apparent suicide of friend Ray Hayward – in real life Deputy Chief Constable David Ainsworth – found hanged at his home as he faced misconduct allegations. Mr Fulcher’s assistant tells him the senior officer drove a dark-coloured estate, similar to that seen on CCTV near Suju.

Soon after Sian’s disappearance, police found mobile phone records that suggested she was driven not home but to the Savernake Forest, near Marlborough.

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Picture: ITV

Concerned boyfriend Kevin Reape, played by This Country star Charlie Cooper, repeatedly tried to contact Sian and reported her missing shortly before 10am.

Over the following days thousands of people joined the search for missing Sian. An anonymous well-wisher pledged a £20,000 reward and the girl’s tearful family told a press conference: “We all want to know where Sian is and we want her home safe and well.”

Halliwell was already under police suspicion and on Thursday, March 24, five days after Sian’s disappearance, detectives swooped on the cabbie in north Swindon.

Against the advice of his deputy, senior investigating officer Mr Fulcher diverted officers accompanying Halliwell from Gablecross police station to farmland near Uffington, where search teams were combing the countryside for missing Sian. Halliwell took police to the place he had buried Sian, before asking Mr Fulcher: “Do you want another one?”

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That question, which took the pair to a field in Gloucestershire where Halliwell had buried the remains of Becky Godden-Edwards, would prove to be a significant moment for both men. The breach of police evidence rules, it would later see a High Court judge rule the cabbie’s countryside confessions inadmissible and result in a police panel finding Mr Fulcher guilty of misconduct.

The author of A Confession, Jeff Pope, called the decision not to take Halliwell into police custody a moral conundrum. Karen Edwards, Becky’s mum, last week backed Mr Fulcher – branding him a hero.

Halliwell is currently serving two life sentences. He was convicted of the murder of Sian O’Callaghan in 2012 and Becky Godden-Edwards in 2016.