Delia Balmer spent four days as the hostage of her murderer boyfriend, only to survive his axe attack – and get him put away. The team behind a challenging drama about her horrific experience tell all

Nick Stevens is remembering the first time he met Delia Balmer, the only known survivor of serial killer John Sweeney, whose story he wanted to dramatise. The meeting went very badly. “She was half an hour late,” recalls the screenwriter. “She wouldn’t look me in the eye. She was very agitated and angry.”

Even though Stevens was familiar with the sensitivities of true-crime stories from his previous dramas In Plain Sight and The Pembrokeshire Murders, this was extreme. But he soon realised that the behaviour of Balmer – played by Anna Maxwell Martin in new drama Until I Kill You – reflected the PTSD she suffers due to Sweeney. In 1994, after she ended their three-year relationship, he held her hostage tied to a bed for four days and later attacked her with an axe, as recounted in her memoir Living With a Serial Killer, on which the show is based.