AS the case against Christopher Halliwell for the murder of Becky Godden-Edwards crumbled, the police officer who had secured a confession from the killer cab driver was accused of sailing close to the wind.

In this evening's episode of A Confession, the action moved from the police station to the courtroom as barristers wrangled over whether Halliwell’s countryside admission to the killing of Sian O’Callaghan and 20-year-old Becky could stand up in court.

The ITV drama is now over halfway through, with only two episodes left in the six-part series.

It re-enacts the investigation into and prosecution of Halliwell, beginning in 2003 with the disappearance of sex worker Becky Godden-Edwards.

Tonight, we saw Halliwell’s barrister – played by Inspector Lynley actor Nathaniel Parker – trap detective Steve Fulcher into appearing to describe police evidence rules as a loophole.

The lawyer argued that Halliwell should have been taken to a police station and allowed to consult a solicitor as soon as officers – thinking he had kidnapped Sian – had asked him some quick, urgent questions.

That Mr Fulcher, played by Hobbit star Martin Freeman, had had Halliwell driven to Barbury Castle for a second interview constituted a breach of the Police and Criminal Evidence Act – PACE – rules. It was during this second interview that Halliwell confessed to killing Sian and led police to Becky’s body, buried in a Gloucestershire field.

“Your own detective inspector, Kirby, told you it was not a good idea to conduct an additional urgent interview yourself with Halliwell, didn’t he?” Halliwell’s QC asks Mr Fulcher, stood in the witness stand. “Yes, he did.”

“He said you’d be sailing very close to the wind, in terms of a breach of PACE.”

As the episode ended, we saw that manoeuvre threaten to capsize Mr Fulcher’s career – despite Halliwell receiving a 25-year prison sentence for murdering Sian O’Callaghan. The detective superintendent is told that a complaint against him, made by Becky’s dad, has been referred to the police complaints watchdog. “They’re going to charge you. Gross misconduct,” Mr Fulcher is told.

In 2014, a disciplinary board would find Mr Fulcher guilty of gross misconduct - but allowed him to keep his job. The detective resigned from Wiltshire Police later that year.

A Confession continues on Monday, September 30, at 9pm.