The shocking true story behind ITV's Until I Kill You

ITV drama Until I Kill You tells the true story of nurse Delia Balmer's terrifying experience with her boyfriend John Sweeney, who she came to realise was a serial killer.

The four-part TV series stars Anna Maxwell Martin and Shaun Evans as Balmer and Sweeney, and shows the early days of the pair's relationship to their horrifying final encounter.

Until I Kill You is based on Balmer's memoir Living with a Serial Killer, which was released in 2017.

The true story of Delia Balmer and John Sweeney

Balmer met Sweeney in a Camden Town pub in 1991, the pair instantly hit it off and found they had a mutual love of travelling. Sweeney told Balmer he worked on construction sites in Germany, and as he seemed well-travelled and exotic she was interested in him.

The early days of their relationship was positive, Sweeney called her regularly and gave her flowers, but it was when their relationship began in earnest that Balmer realised his early affection was all a manipulation. He became increasingly controlling and soon subjected her to horrific abuse.

Read more: Love bombing: The manipulative relationship tactic to know about

Writing in her memoir, per Ham & High, Balmer said: “Subtle changes happened slowly. I should’ve known something was wrong but I am too soft and trusting. I’d never come across anything like it before.”

Three years into the abusive relationship Balmer was desperate to get out but feared what would happen if she did. In 1994 Sweeney briefly left Balmer, she changed the locks and asked police to investigate the flat where a bag was discovered hidden behind a bathroom panel that she describes as "a body disposal kit meant for me".

Balmer was terrified of Sweeney returning, but despite her warnings to police Sweeney broke into her home and held her hostage for four days. Sweeney tied her to a bed and threatened her with a gun, before telling her about murdering his previous girlfriend Melissa Halstead and two German men in Amsterdam.

Halstead's remains had been found in a canal in Rotterdam in 1990, but police had not yet found a suspect. Of his confession, Balmer wrote in her book: "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."

Read more: ‘She still sleeps in the bed she was tied to’: Anna Maxwell Martin takes down a serial killer (The Guardian, 9-min read)

Sweeney eventually fled the flat, and Balmer went to police to tell them what had happened. They arrested Sweeney for holding her hostage, but he was let out on bail.

Balmer was terrified, and her fears came true when she returned home one day to find Sweeney with an axe. He attacked her and cut off one of her fingers, a neighbour was able to help Balmer by striking Sweeney with a baseball bat and he went on the run.

In 2017, Balmer told The Mirror of the experience: "On the doorstep, I saw this finger fly through the air, and I thought: 'That’s it. I don’t want to live anymore. I don’t want to live in this anger and pain'. So I wasn’t happy when I woke up in an intensive care unit. I thought: 'Oh no, I am alive. Now what hell?'"

Balmer was severely injured during the attack, she had torn tendons and a broken arm as well as a missing finger. She told The Mirror that she is still living with the mental and physical pain of that experience, saying: “Even after all these years, the scars and the pain are a constant reminder of my life with a serial killer."

Sweeney was arrested in 2001 and was given four life sentences for the attempted murder of Balmer. In 2010, whilst serving this sentence, he was charged with Halstead's murder as well as the murder of Paula Fields, who he had killed whilst on the run.

He was found guilty of both crimes in 2011 and sentenced to life imprisonment. Authorities believe he may have killed three more women between 1970 and 1990 per the BBC.

Until I Kill You airs on ITV1 every night at 9pm until Wednesday, 6 November.

For support contact Women's Aid, Victim Support, The Survivors Trust or Survivors UK (for male victims of sexual assault). Or you can contact the 24-hour freephone National Domestic Abuse Helpline, run by Refuge, on 0808 2000 247.

For confidential emotional support contact The Samaritans at any time by calling 116 123 or emailing jo@samaritans.org