Is Christopher Halliwell, 48, really likely to ‘walk free’ when he has served the 25-year minimum term he was given for murdering 22-year-old Sian O’Callaghan, as one newspaper reported on Saturday? Sentencing him to life imprisonment a day earlier, Mrs Justice Cox told him: ‘If you are eventually released on licence you will remain on licence for the rest of your life.’

The newspaper reporter was trying to make the point that Halliwell might never have faced the prospect of release if he had been convicted of two murders involving ‘sexual conduct’. In such cases, the starting point is a whole life order, unlike the tariff for a single sexually motivated murder which starts at 30 years. And Halliwell, who received a discount for pleading guilty, could indeed have been convicted of two such murders if only the judge had let a second charge proceed to trial.

Leave aside the possibility that Halliwell may face further charges in prison. Ignore the slim prospect that the Parole Board will think it safe to release a suspected multiple killer after 25 years, even though Halliwell will then be in his early 70s. The story, for most news outlets, was that another murdered woman, Becky Godden-Edwards, had been ‘denied justice’ because Halliwell had not been convicted of murdering her too.

Godden-Edwards was not denied justice. Neither was her mother, Karen. Justice exists to ensure the guilty are convicted and the innocent are acquitted. If that means some offences do not result in prosecution and punishment, then it is the price we pay to ensure innocent people are not wrongly convicted. O’Callaghan was abducted as she walked home from a nightclub in Swindon. Within three days, CCTV evidence established that Halliwell, a taxi driver, was a suspect and he was put under surveillance.

Detective Superintendent Steve Fulcher of Wiltshire police, who was in charge of the investigation, suspected that O’Callaghan was dead while hoping she might still be alive. That, he thought, justified using what is called the ‘urgent interview’ procedure in the codes of practice made under PACE, the Police and Criminal Evidence Act 1984. Code C.11.1 may permit an arrested person to be interviewed before being brought to a police station if delaying the interview would cause physical harm to others.

Halliwell was arrested by junior officers who properly conducted a brief urgent interview with him in a police car. The defendant gave nothing away and they set off for the station. Before they arrived, Fulcher diverted the car to what was thought to have been the scene of the crime so that the detective could ‘continue’ the urgent interview himself. After speaking to Halliwell for nine minutes in contravention of the PACE codes, Fulcher persuaded him to show searchers where O’Callaghan’s body was buried.

At this point, Fulcher told Halliwell he was to be arrested. But Halliwell then told Fulcher, in effect, that he could take him where another body had been buried. That enabled the police to find Godden-Edwards, whose disappearance in 2004 had never been reported to them. Finally, Halliwell was arrested and driven to the police station, where he refused to answer questions on legal advice.

The murder suspect had been denied access to a lawyer for four hours. He had not been reminded of his right to silence. The judge concluded that his confessions were inadmissible because they were made under oppression and that the evidence of what happened between his detention and arrival at the police station could not be used in court because the PACE codes had been breached. The CPS did not appeal.

‘As soon as he began to talk about another offence it is clear that he should have been cautioned,’ Cox said. ‘There should have been no further discussions about it and the defendant should have been taken to the police station.’ Since there was no other evidence linking Halliwell to Godden-Edwards, he could not be tried for her murder. But Fulcher had already told reporters that Halliwell had admitted two murders. His lawyers argued that it was therefore impossible for him to be tried fairly for O’Callaghan’s murder alone on the remaining, untainted evidence.

It is fortunate that Cox disagreed, finding that Fulcher had not been acting in bad faith. ‘I do not consider that this officer’s misjudgment, serious as it was, is properly to be categorised as an assault on the integrity of the criminal justice system,’ she said.

Wiltshire’s current chief constable had told Cox that Fulcher’s decision to ignore the PACE codes was a ‘brave’ one. It was not. It stopped one trial and jeopardised another. Chief constables may be getting younger but they should be old enough to remember the miscarriages of justice whose recurrence PACE was intended to prevent.