A version of this article was first published in March 2017. Line of Duty season six will conclude on BBC One on Sunday 2nd May at 9pm.

Over the past few weeks, Line of Duty has once again transfixed the nation. An incredible 11 million viewers tuned in to watch the penultimate episode of series 6, making it the most-watched episode of TV in this country since 2008, and social media is abuzz with speculation as to the identity of 'H' or 'The Fourth Man' – which looks like it will finally become clear during Sunday night's finale.

Set in a fictional police force – the first series was shot in Birmingham, the subsequent five in Belfast – Line of Duty follows the activities of AC-12, a small anti-corruption unit. Since first arriving on BBC2 in 2012, it has swiftly established itself as one of the finest of police procedurals, earning it a promotion to BBC1 from series 4 onwards.

The three leading cast members – Superintendent Ted Hastings (Adrian Dunbar), Steve Arnott (Martin Compston) and Kate Fleming (Vicky McClure) – are currently investigating allegations against Jo Davidson with the same forensic attention to detail that’s become a hallmark of the show.

Speaking in 2017, Mercurio said, “I think it’s important to be as close to the right procedures as possible. The starting point is with our advisers and I always get excited if we can use something I haven’t seen in any other series.”

Adrian Dunbar and Martin Compston in Line of Duty season 6
Adrian Dunbar and Martin Compston in Line of Duty season 6 (BBC)

With an eclectic background in medicine and in the RAF before his current television career, Mercurio reckons that we are in a golden age of crime dramas. He is too diplomatic to single out current shows – “there are so many very good ones” – but credits the American series Hill Street Blues, which he watched in the 1980s, as an inspiration in the way it broke away from previous conventions. He also singles out an earlier BBC television drama about anti-corruption cops, Between the Lines, which starred Neil Pearson and won a Bafta in 1994.

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The history of the delicate business of investigating corrupt police officers is a long one. When Sir Robert Mark arrived at Scotland Yard as its new commissioner in 1972, he famously announced that it was his intention to “arrest more criminals than he employed”.

This was an era when a “little firm within a firm”, as the bent cops cheerfully described themselves, operated in Britain’s largest police force and corruption, in the shape of accepting back-handers and planting evidence, was rife. In 1973, three members of the Met’s drugs squad were jailed at the Old Bailey and told by the judge, Melford Stevenson, that they had “poisoned the wells of criminal justice... and not the least grave aspect of what you have done is provide material for the crooks, cranks and do-gooders who unite to attack the police whenever the opportunity occurs.”

Worse was to follow. In 1976, a further scandal concerning the “Porn Squad” in Soho led to the departure of more than 60 Scotland Yard detectives. Gilbert Kelland, the then assistant commissioner of the Met, said later, “We strongly believed that, for the eventual benefit of the force, the crow of corruption had to be nailed to the barn door to convince and remind everyone of the need for positive action and eternal vigilance.”

Twenty years later, another commissioner, Sir Paul Condon, told a House of Commons select committee in 1997 that there could still be more than 200 wrong ’uns on the payroll. “I honestly believe I command the most honourable large city police service in the world,” he said. “However, I do have a minority of officers who are corrupt, dishonest, unethical... They commit crimes, they neutralise evidence in important cases and they betray police operations and techniques to criminals... they are very difficult to target and prosecute.”

So how are those “rotten apples” spotted and plucked? The names of anti-corruption branches have changed over the years. In the Met, what was once called A10 was replaced by CIB1 and CIB2 and is now the less snappy Directorate of Professional Standards.

Other forces also have their own Professional Standards Departments, although smaller, provincial forces may pool their resources. The nicknames for these units have changed, too. Once they were the “Ghost Squad” or the “Untouchables” and their detectives known as the “rubber heels” because of the unobtrusive way in which they had to operate. In their less effective days, some were dismissed by cynical detectives as “the muppets”.

What do their fellow officers think of them?

“CID officers used to be reluctant to apply for this department because it wasn’t seen to be the right thing to do and was regarded as a betrayal,” says one former Flying Squad detective, who is a fan of Line of Duty. “Uniformed officers did apply, which resulted in an imbalance of investigators, built on a false premise that uniform were out to get the CID – and no decent detective would want to be part of that. But in the 1990s, the culture started to change and therefore the job was no longer seen as a betrayal of their colleagues. It’s now accepted by all that corrupt officers have to be weeded out, so it’s seen as a good career move.”

Vicky McClure and Kelly Macdonald in Line of Duty
Vicky McClure and Kelly Macdonald in Line of Duty (BBC)

A major challenge for any anti-corruption squad is persuading victims of police skulduggery to trust them. When I was The Guardian’s crime correspondent in the 1990s I investigated allegations that some officers at Stoke Newington police station in north London were planting evidence on people and dealing in drugs themselves. After interviewing some of those making the claims, I was told by a lawyer that a senior officer from what was then called CIB (Complaints Investigation Bureau) wanted to meet.

It turned out that CIB had already mounted a major inquiry into Stoke Newington, code-named Operation Jackpot, and they knew I was talking to people, some of whom were in prison, and many of whom were very wary of talking to coppers – any coppers. Could I pass on the word to them that this was a serious inquiry and their confidences would not be betrayed? They also wanted to trace a witness known then only by his street name. I believed that it was a genuine attempt to nail the guilty (like those Line of Duty investigations) but those making the allegations remained understandably cautious.

A small number of officers were eventually prosecuted and jailed and 13 people had their convictions quashed at the court of appeal and paid more than £500,000 in damages by the Met police. It was a reminder that, when the police investigate the police, not everyone is convinced they will do so without fear or favour.

Martin Compston stood wearing a police lanyard in an office.
Martin Compston as DS Steve Arnott in Line of Duty season 6 ep 1 (BBC) BBC

Former Detective Superintendent Graham Satchwell, who was himself investigated and cleared by several internal inquiries, says that “the more ‘results-driven’ detective officers saw internal investigation people as – at best – a nuisance”.

Satchwell, author of the memoir An Inspector Recalls, adds that “the abiding memory of those who investigated me is of incompetence and stupidity. Those who run internal complaints departments are always close to the most senior ranks, so whatever they do is authorised. Invariably now, such work attracts ambitious officers.”

Certainly allegations of malpractice are taken much more seriously now than was once the case. Back in 1955, when Superintendent Bert Hannan produced a report on corruption in London’s West End, the then Met commissioner, Sir John Nott-Bower, went to the station concerned and stood on a chair to reassure the troops that he did not believe a word of it. It would be a foolhardy commissioner or chief constable who did the same today.

So in Line of Duty, Hastings, Arnott and Fleming have the daunting task of reassuring us that they are more than capable not only of catching bent cops but of nailing that crow of corruption firmly to the barn door.

For more Line of Duty coverage, read our Line of Duty episode 6 recap, our ode to Carmichael and our explainer on Jo Davidson’s ‘father’.Line of Duty season six concludes this Sunday at 9pm on BBC One – take a look at the rest of our Drama coverage, or check out our TV Guide to see what’s on this week.