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Norton, unsurprisingly, isn't too sympathetic to Royce in real life. "Catherine's the bigger, better person and so much stronger by choosing not to kill Tommy," says Norton. "His life's ruined, he's off to prison and he might not even make it – he's had a month slowly dying of septicemia. I watched all the other episodes go out live but I'll be at an awards show on Tuesday night. It'll be a weird feeling to look at my watch at 9.45pm and think 'So, 8 million people are currently wishing me dead.' Even my dear old mum said she's found herself standing up, screaming at the telly – and then suddenly remembered that it was her son."

Norton believes the final showdown posed an intriguing moral dilemma. "One of the show's strengths was that the audience got totally on-board with Catherine's revenge mission. There was that moment when she's standing over him and he's screaming 'Kill me!' She picks up the fire extinguisher and you think, "Go on, kill him!" Hopefully viewers will question why they suddenly got that bloodlust."

DISCUSS: Has Happy Valley become too violent?

Norton says he relished the chance to let the villain's mask slip, as he hesitantly tries to bond with the son he never knew he had. "The production team put me in touch with a psychologist called Costas, who works at the Priory in Manchester and we spent a long time discussing the scenes. On the surface, Tommy's totally callous and detached, with no empathy at all, a classic trait of a psychopath, but what you realise is that nearly all psychopaths have been abused and incredibly badly treated as a child and their behaviour is borne out of a desperate need for control. They'll do anything to avoid being that helpless mistreated infant again.

"In Tommy's head, the beatings and rapes are necessary for him to survive. The moment he starts to lose that control is when he's stabbed in the kitchen last week. He said as much: 'The game's up, it's over.' From then on, he started to return to that desperately vulnerable child. He starts drinking milk, bursts into tears and huddles up in the foetal position. It was fascinating to let his humanity bubble through like that. Ryan was the first person to pierce Tommy's armour in years."

On paper, it had all sounded so cosy: a new BBC drama with the twee title of Happy Valley, created by that nice Wainwright (who wrote the romantic Last Tango in Halifax), starring the much-loved Lancashire (aka ditzy Rover's Return barmaid Raquel) and set in the picturesque Yorkshire town of Hebden Bridge.

In reality, it was anything but cosy. This uncompromising six-parter took in drug abuse, depression, cancer, kidnapping, rape, murder, gangland killings and suicide. Yet viewers lapped it up, with 7m of us glued to our screens in suspense every week, making Happy Valley TV's current top-rated drama.

"It's always a joy when people engage with something you've written but I've been blown away by the reaction," says Wainwright, quietly but proudly.

"It seemed to be a genuine word-of-mouth hit. Friends kept telling me that everyone was talking about it at work on Wednesday mornings." As a prolific screenwriter who produces several series per year, the 50-year-old tends to be chained to her desk and laughs: "I don't get out much myself, so it was very gratifying to hear."

As for the discovery of Norton: "James did an extraordinary audition," says Wainwright. "We saw 20 fantastic actors but all of them played up the evil. James did it quietly and subtly, but with real menace." Norton's also been cast in ITV period crime drama Grantchester, coming up this Autumn, which should further boost his profile when he shares top billing with Robson Green.

Norton says he knew instantly he had to do the role. "The script leapt off the page. I couldn't stop reading. Suddenly hours had passed and I'd devoured the whole thing. Sally's at the top of her game."

The brutal fight scene he had to do with Lancashire led to viewer complaints and criticism from Mediawatch. "I was a little surprised by the controversy," says Norton. "It wasn't gratuitous, it was a key part of the story."

"I think it drew attention to the series, to be honest, and maybe made more people watch it," adds Wainwright. "But the overwhelming response has been that we got it right. Violence against women is part of life and if you don't show it, you're complicit in pretending it doesn't happen. I'll be interested to know if the people who complained about Tommy beating up Catherine two weeks ago complain about it being the other way around last night."

Success means a second series is inevitable, although Wainwright says nothing is yet confirmed. "I always secretly hoped it could run and run. I can't say too much but Catherine would be the constant. And there are other characters that you might see again." They're unlikely to include Royce, though. "Not unless they stage some slightly ludicrous prison break," laughs Norton. "Although one interesting slant could be that Ryan now knows that Tommy's his dad. Who knows?"

READ: DO WE REALLY NEED MORE TV CRIME DRAMAS?