Until I Kill You stars Anna Maxwell Martin as Delia Balmer

Melissa Halstead’s remains had been found in a Rotterdam canal the year earlier, but it took nearly twenty years to connect them to the Liverpool carpenter.

In the meantime the Royal Free nurse endured an abusive relationship with the killer, including a horrifying ordeal when he trapped her in her Kentish Town flat, held a gun to her head, and threatened to kill her.

He went on the run, later turning up with an axe and a knife to attack her outside her Leighton Grove home before disappearing again.

Delia Balmer met John Sweeney in the Hawley Arms Camden in 1991 and they started a relationship. (Image: Newsquest archive)

The story of how she survived, which pays tribute to Delia's resilience while spotlighting failures of the criminal justice system, is told in new ITV drama Until I Kill You starring Motherland and Line of Duty actor Anna Maxwell Martin.

She plays Delia opposite Shaun Evans' Sweeney, who was finally caught in February 2001 when the body parts of his girlfriend Paula Fields were found in the Regent’s Canal in Camden.

Nick Stevens' true crime drama is told across four episodes, with Maxwell Martin's Delia narrating her traumatic journey through the police and courts system.

Anna Maxwell Martin plays Delia Balmer in ITV1's Until I Kill You opposite Shaun Evans as John Sweeney (Image: ITV Studios)

Maxwell Martin said it was Delia's character that drew her to the drama: "She’s unlike anyone I’ve ever played before. It was really liberating to play someone who is unapologetically herself and gives herself licence to express everything she’s thinking and feeling. Very few people are like that."

The actor baulks at overplaying the hardship of performing the drama's darker scenes: "It's not like doing a 12-hour shift as a nurse, so I get embarrassed to say that. It’s not a nice headspace to be in, but I get to step away from it. You have to keep a firm grip on the fact that it is not your reality – very sadly it was someone else’s reality and it was really difficult for her.

"It affects you a bit, you wouldn’t be doing your job properly if it didn’t, but you’ve get a firm grip on yourself, go home and be thankful that you’re not living any of those things."

The dismembered remains of another of John Sweeney's girlfriends Paula Fields were dumped in holdalls in The Regent's Canal and recovered in 2001. (Image: PA)

Maxwell Martin chose not to meet the real Delia while preparing for the role, although they met in Wales during a set visit.

She adds: "Whatever story you’re telling, you must be respectful of the people you’re playing. We're trying to say, “Wow look at this amazing woman” – she didn’t take any crap, she’s so resilient and fiery, such an individual, let’s celebrate that rather than showing more gratuitous violence against females on TV.

"I think we’re all a bit bored of following predatory male killers around, I didn’t really want to focus on the abuse. We’re getting inside the mind of a woman who went through a lot of complex things.

"I was interested in the brutalising system Delia became a part of – the judicial system, the police system. Lots of people are missed all the time and Delia was. She is an incredibly strong, capable person and I hope we challenge the idea that victims of violence are wobbling jellies in the corner. Delia was very rarely cowed by people, she had incredible resilience."

The four part ITV true crime drama shows how Sweeney's relationship with Delia became increasingly abusive. (Image: ITV Studios)

Maxwell Martin believes the series would not have been made unless it was a true story.

"There are aspects of Delia’s character that are quite extreme, if I had written this character and taken the drama to a broadcaster they would have told me to tone it down. But there was so much freedom when filming this, because Delia is an exceptionally interesting person and this is a true story."

The ITV series is based on Delia’s book, Living with a Serial Killer.

Released in 2017, at the time she was living in Belsize Park and told the Ham&High she was still suffering from Sweeney's 1994 attack and felt let down by Kentish Town Police who failed to take her warnings seriously.

“The last thing I remember was seeing my finger fly through the air, I awoke in the Royal Free and all I wanted to do was die, I didn’t want to live any more in this body, in anger and pain. I had torn tendons, a broken arm, the slightest touch was sensitive," she said.

A Met Police handout of John Sweeney who in 2011 was finally jailed for the murders of Paula Fields and Melissa Halstead. (Image: PA)

"People ask am I afraid, but my worst fear was before he cut me up, when I knew what he was going to do and that Police weren’t going to help."

The book starts in 1991 with Sweeney’s unsettling stare, and offer to buy her a drink.

“I was bored, alone, the Hawley Arms had the best jukebox and I liked people watching. I think now that he smelled me, there was something there he could get into.”

Early on there were flowers, Sweeney rang when he said he would, and fixed her window.

“I thought of him as a bohemian, like me he travelled a lot. He seemed so nice and tended to a stutter. But now I know it was all a big show. He was playing his game and I was stupid because I was alone.”

Sweeney moved into her flat pledging to “be no trouble,” but his behaviour became increasingly aggressive and controlling. When he viciously beat a man in Germany she tried to end the relationship.

"Once I saw what he was like, I tried to get rid of him. He kept saying he would go, but I knew he wouldn’t. The police just saw me as a domestic violence victim. I knew a restraining order wouldn’t work on John and it would put me in more danger."

Finally Sweeney left, leaving behind a portfolio of violent drawings and a murder bag hidden behind a bath panel. Balmer describes her terror that he would return, getting the lock changed, and the police to search the flat.

But he broke in through a bathroom window, held a gun to her head, tied her to the bed, and told her about murdering Melissa.

“It was as if he could read my thoughts, I knew not to scream, there was no telling what he might do, he had this evil look in his eyes.”

Held in Pentonville for holding Delia hostage, he was fatally let out on bail.

“I begged them ‘don’t let him out or he will cut me to bits’. I told them about Melissa, showed them his drawings but they let him out. I was in such a state, so scared," Delia told the Ham&High.

The Domestic Violence team assured her Sweeney wouldn’t break his bail conditions, but on December 22, 1994 as Delia returned from a nursing shift, he was waiting with an axe and knife.

“I was on the doorstep he had the axe over his head, I shut my eyes waiting for it to come down, then the neighbour’s son hit him with a baseball bat and he ran off with the axe."

After seven years on the run, Sweeney, now living in Finsbury Park, was arrested in 2001 following the discovery of Paula Fields' remains. Despite not trusting the system Delia bravely testified against him and he was given four life sentences for attempted murder.

Following a cold case review, police finally identified Melissa’s body and paired it with the Fields case, and in 2011 Sweeney received a whole life sentence for both murders.

Now 74, Delia wants to get the truth out about what happened to her but still suffers PTSD from her experiences. (Image: ITV Stuidos)

Now 74, Balmer's main concern for the TV series is to "get the truth out by whatever means."

She still suffers from depression and anxiety, does ballet twice a week and travels when she can: "I remain an angry person. Sweeney was let out on bail and the police gave me insufficient protection before his final assault. I am afraid of life, afraid of the future, a compulsive worrier. I am stuck I cannot move on. I live for travel it has become my only escape from reality."

A companion documentary Until I Kill You The Real Story will be broadcast on ITV.

Screenwriter Nick Stevens, who spent two years gaining Delia's trust and interviewing her, said: “She was unlike anybody I'd ever met before.

"Delia knows she can be a difficult person, partly due to her own unique wiring, partly due to the PTSD she still suffers. Once I’d got to know Delia – and spoken to some of the police officers who handled her case – I began to realise the extent to which Delia’s personality had negatively impacted on her dealings with the Criminal Justice System whose job it was to protect her.

"She was not always a compliant or cooperative victim. She was often chaotic and angry. Ideally, a victim’s personality should not affect the quality of the justice they receive, but that is exactly what happened in Delia’s case."

He adds: "Delia is, though she would never admit it, a hero. A survivor whose determination to pursue the good in life – to dance, to travel, to live - is ultimately greater than her demons."

Until I Kill You is on ITV1 and ITVX from 3rd to 6th November at 9pm followed by the companion documentary Until I Kill You: The Real Story on 7th November at 9pm.