Inspector George Gently faces the final curtain with a moving penultimate film

But the main reason Gently is so furious is that Bacchus was implicated in the failings of an old case which has led to a miscarriage of justice. He had doctored a witness statement and – worse – had pressured ACC Nicholls (Lorcan Cranitch) to drop the new investigation.

Perhaps worse than his anger, though, is Gently’s deep disappointment in the young man.

Dodgy 1970s coppering is the last thing Gently wants or expects from his sidekick, and in the case of the murder of Alistair Liddell (whose body is found in a factory waste tank eight years' after his disappearance) that is what he did – fitting evidence to suit his suspicions that his wife, Polish immigrant Eve (Annamaria Marinca, below) was the killer.

But of course, the key evidence – the young daughter’s sighting of her mother cleaning up blood – didn’t point to Eve’s guilt. She was not in the house at the time of the killing, and the perpetrator turned out to be Emma Rigby’s Betty Platt. She had challenged Alistair about her boyfriend Robert (Robert Lonsdale) who had been burned in a fire caused by Alistair’s incompetence and then sacked, and matters took an ugly turn.

Instead of addressing the grotesque injustice, Alistair tried to rape her – so she killed him. Bacchus and the rest of the investigators had pinned the blame on Eve, who was too ashamed to admit the truth because she was having an abortion on the night of the murder; a termination she sought because she couldn’t bear having a child with her abusive husband.

Domestic violence and sexual cruelty were clear themes of the episode, as was racial injustice.

It was clearly suggested that Eve was set up for the crime partly because she was foreign. Their neighbour who claimed to have overheard them arguing said she wasn't their “sort”, while Alistair, an unsavoury piece of work, was.

Ah, the 1970s. An age when old attitudes are dying hard. But Eve was eventually released to the sight and sound of a resounding feminist demo led by her formerly estranged daughter and talented artist Marion (Victoria Bewick, below).

Gently smiled at the reunion, the old and the new worlds met, and you sensed that whatever horrors these dark times could unleash there were at least men like George there ready to fight them.

But for how long? All will be revealed in the second and final film of this run, which is expected to air soon.

It won’t air next Sunday - partly, RadioTimes.com understands, because the story is based on a Labour MP which would be too sensitive show just four days before the election. But things are set up nicely for his final send off.

At a screening of the penultimate episode, Shaw said he was pleased that his detective was getting a proper farewell.

“It’s been such a popular show and the characters are so well drawn… that you need a proper coda," he said. "You need to end the symphony with a striking chord.”

As for Bacchus himself, he needs to get his house in order, cut out the drinking and win back Gently’s trust. But would Lee Ingleby be up for a spin-off? How about a new series called Inspector Bacchus once Gently slips off to retirement?

“It’s a hard one; it’s hard to see beyond this show. I love John, he’s always been a favourite character. You wouldn’t do it if it was just a boring character. I don’t know if I’d have carried on. He was just a pleasure to play with these guys, but it would seem odd.”

Future spin-offs aside, we can look forward to the final outing where, we're told, Gently faces up to the emotional impact caused by the death of his beloved wife Isabella, murdered by a notorious gangster just before the start of the series. As in all George Gently episodes, it won’t be easy viewing. But it will be fascinating and compelling.