Until I Kill You writer Nick Stevens on his duty of care to the survivor of a serial killer

A man offers to buy her a drink. She is bored, so thinks, “Why not?” They sit down. John Sweeney is a bit of a carefree hippy, a charismatic carpenter from Liverpool who often works in Europe. He quickly moves into her flat.

Pretty soon, the love bombing stops. Sweeney becomes coercive and controlling, but when Balmer asks him to move out, he ties her hands and feet to the bed that he built for her. He threatens to cut out her tongue if she screams. He confesses to the murder of American model Melissa Halstead, who went missing in 1986.

When, some days later, Sweeney releases Balmer, her colleagues urge her to go to the police. When the cops come to her flat, she shows them Sweeney’s violent drawings of Halstead and tries to convince them that Sweeney is a killer. They dismiss her claims as those of an eccentric woman.

Sweeney breaks into her flat. The friend Delia was due to meet rings 999. The police find a “killer’s kit bag” with a hacksaw blade, rubber gloves, tarpaulin, duct tape and so on. Sweeney is charged with false imprisonment and ABH and then released on bail.

On 22nd December 1994, Balmer is returning home after finishing her last nursing shift before Christmas. Sweeney looms out of the darkness and attacks her on her doorstep with an axe. She is on the brink of death when a neighbour intervenes, saving her life.

Sweeney goes on the run, Balmer slowly rebuilds her life and seven years later he returns. She is persuaded to testify against him in court and in 2011 he is finally given four life sentences for viciously attacking Balmer and for murdering both Halstead and a Liverpudlian woman called Paula Fields. He may also have killed and dismembered other women across Europe.

Why, you might ask, does this kind of horrific, graphic story make such compulsive television – and why is there such an enduring appetite for it? Stevens should know: he’s written about serial killers before, in hard-hitting dramas In Plain Sight and The Pembrokeshire Murders. “People refer to true-crime dramas as modern horror stories, but they aren’t the escapist entertainment traditionally offered by horror films.

“I think that, by virtue of their truth and of the strangeness of the stories being told, they are utterly compelling in a way that fiction is not.”

Stevens, who has the enviable ability to “just switch off” after spending all day writing fictional iterations of true-life killings, puts in a huge amount of work to ensure that each story he adapts for the small screen is handled with sensitivity, a duty of care, and a lack of sensationalism.

He picked up Balmer’s memoir, in a bookshop in 2017. “As soon as I realised it was from the perspective of a woman who had survived, I was hooked. She describes her distressed, unconventional behaviour while standing in the witness box at the Old Bailey, giving evidence against Sweeney. I’d never read anything like it before.”

He arranged to meet Balmer at her literary agent’s office. “I was taken aback; Delia was agitated and hostile. She wouldn’t look me in the eye. She was so full of mistrust and anger that it was almost impossible to have a conversation with her. She wasn’t insulted by my overtures; she was up for it from the word go, but she wanted to be sure it was an accurate depiction of her story.”

Stevens spent the best part of two years talking to Balmer and eventually won her trust, but it took time and patience. Among other things, he had to explain the practical realities of adapting a story covering two decades into four episodes of commercial, one-hour TV.

“I sent her the scripts once they were written and then changed whatever I could without damaging the narrative structure too much. It was mostly details because Delia is a perfectionist. And the good news is that she’s made some money from the series, so she can do a bit of travelling and have ballet lessons every day.”

Balmer made two visits to the set of Until I Kill You, one in Swansea and the other in London, near to where she now lives. “Delia came to the shoot with a mate. Anna [Maxwell Martin] is a genius, but when she’s playing a real person, she doesn’t want to meet them as part of the research. She was very nice to Delia, but not in a ‘I want to get to you know you so I can play you better’ way. And Delia didn’t meet Shaun [Evans] for obvious reasons. The production team were very deliberate about that.”

A standalone companion documentary, Until I Kill You: The Real Story, is also narrated by Maxwell Martin and includes interviews with Balmer, who is now 74. She recalls that when she was told the only way to convict Sweeney was through her testimony, she felt as though she was the guilty one. “They made a mockery of all the scars and pain I have.”

Balmer has now seen all four episodes of Until I Kill You and was, Stevens says, “laughing a lot during the screening”. Partly, one imagines, as a result of extreme PTSD, but also because “it’s the way she deals with it all, this brilliant woman who went through unimaginable trauma”.

There is, it seems, a thin silver lining to this horrendous story about the banality of evil and the failure of the legal system to believe women. “My wife and I regularly take Delia to our favourite restaurant,” says Stevens with a grin. “She is part of our lives now. I think the process of making this drama has been a very positive experience for her and she’s a different person to the one I first met.”

Until I Kill You will air on ITV1 and ITVX from Sunday 3rd November at 9pm on ITV1.

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