Surviving the depravity of a serial killer is a badge of curious distinction and one that few hold.

Delia Balmer survived. But, beaten with a wooden axe, hacked with a rusty knife and left for dead on her own doorstep by a man considered one of the most dangerous killers in the UK, means she carries scars, outwardly and inwardly, that will never fade.

As she says: ‘I’m a perfectionist, but my life is the opposite of perfect’.

There’s the absence of the finger that John Sweeney, the man with whom she shared a turbulent three years of her life, severed in the frenzied attack, days before Christmas, back in 1994.

But that is, one suspects, nothing to the constant pain of the other injuries he caused: knife wounds to her breast and thigh, injuries that very nearly took her life.

Serial killer Sweeney is serving a whole life term at HMP Belmarsh and is forever recorded in the annals of crime as ‘The Scalp Hunter’

As viewers of a compelling forthcoming ITV documentary about Delia and her attacker will discover, this slightly-built woman, now in her early 70s, is both quietly composed and bristling with fury at the events that upturned her quiet life.

‘I’m an angry person,’ she says, matter-of-fact in her delivery. ‘You know I’ve been very bitter and I’m still quite bitter and I don’t like the damage done to this body. It need not have happened.’

So what did happen to Delia Balmer?

The fact that she is around to articulate her rage at what happened to her – she is thought to be Sweeney’s only known survivor – is nothing short of a miracle.

As the retired nurse says: ‘It is the sort of thing you read in the papers or watch in a movie, it doesn’t happen for real. But it did, it happened to me.’

Delia, of course, is now perfectly aware of her unique status on John Sweeney’s charge sheet. A man so depraved, he is serving a whole life term at HMP Belmarsh, and forever recorded in the annals of crime as ‘The Scalp Hunter’, a title taken from the chilling autobiographical artwork that would ultimately bring him to justice.

It was a drawn out and tangled process. It was six years after his attack on Delia, that the dismembered body of a woman was found in London’s Regent’s Canal – it was mother-of-three Paula Fields; it would be another eight years before advances in DNA technology enabled police in Holland to identify another body, that of Melissa Halstead, an American model whose dismembered remains had been found in a Rotterdam canal in 1992.

These are just the known victims, however. Police and criminologists remain convinced that Sweeney, a carpenter, originally from Liverpool, who’d worked on construction sites around Europe for decades, has killed more.

But, as viewers of the documentary and the accompanying drama series, Until I Kill You – starring Anna Maxwell Martin as Delia and Shaun Evans, as Sweeney – will see, the unravelling of the psychopathic trail of a killer starting with Delia Balmer.

Shy and quiet, growing up in Texas, Delia longed to see the world.

Her travels brought her to London where she trained as a nurse and where, in 1991, she found herself at the bar of a pub that has now become famous for quite different reasons – the Hawley Arms in Camden, where Richard Gadd of Netflix’s ‘Baby Reindeer’ worked, and where the late singer, Amy Winehouse, would sink pints and play pool.

Delia was beaten with a wooden axe, hacked with a rusty knife and left for dead on her own doorstep by Sweeney

The dismembered body of mother-of-three Paula Fields was found in London’s Regent’s Canal

When a ‘hippy looking’ man offered her a drink, Delia remembers thinking: ‘Oh well, I’m bored. At least somebody to talk to.’

The man was, of course Sweeney, a divorcee with two children (not that he told Delia this), who said he’d been working in Germany.

‘He sounded so carefree and he could do what he liked,’ she says. ‘I wasn’t looking for a boyfriend. But with me, into travel so much, that was the attraction.’

Knowing what we do now about Sweeney, it might seem bewildering that so many young, attractive women would fall for someone who is still, even behind bars, regarded as a ‘damaged, dangerous and disturbed man’.

But as criminologist Professor David Wilson tells the Mail, Sweeney, has all the traits of a psychopath, including cunning and charm, an ability to hone in on a victim’s vulnerabilities and mirror their passions.

‘One thing I say [to students] is that you imagine a psychopath is going to have horns and a long-pointed tail and that’s not what they are like at all,’ he says. ‘Psychopaths are very seducing. Psychopaths are going to be that bloke who comes up to you in a club and love bombs you so you trust him.’

Sweeney soon won Delia’s trust. She invited him to move into her flat in north London a few months after they met.

On camera, she returns to the street in which she once lived to look at the place where her life nearly ended; she also holds aloft a small pink cushion he bought back from his travels, emblazoned with the phrase ‘Ich liebe dich,’ I love you, in German.

There are other reminders too. The long, narrow table he made for her, under which his name is still imprinted, with the date, and, bewilderingly, the bed he made, in which Delia still sleeps.

Looking back, Delia admits she did have doubts but, ‘tried to make myself believe it was for real’.

An early hint of what was to come, came on a trip to the US where Delia introduced Sweeney to her family, including younger brother Stewart.

On a night at Stewart’s home, drinking wine, came a moment that now stands out.

‘I asked him: ‘Have you ever killed anyone?” says Stewart. ‘I don’t know why I asked it, I just had these vibes about him. He wouldn’t say. He said: ‘The white men taught the Indians how to scalp,’ really strange.’

He also fancied himself as an artist and began hanging up some of his paintings on the walls.

One of Sweeney’s paintings with his arm wrapped around a blonde woman whom he claimed was an ex-girlfriend called Melissa

Delia found them macabre: self portraits of Sweeney with horns on his head, a nude girl tied up, a coffin and another of Sweeney with his arm around a blonde woman. Who was she? An ex-girlfriend, Melissa, he replied.

She said nothing about the strange pictures, but with time came something worse. Sweeney’s behaviour towards her became cruel and controlling.

It culminated in a four-day ordeal of unimaginable horror when, out of the blue, he suddenly flipped one night, and tied her to the bed.

She remembers how he threatened to cut out her tongue if she screamed and waved a gun in her face as he assaulted her.

He also opened up to her, telling her about the ex-girlfriend in the painting, a beautiful photographer who’d disappeared months before Delia and Sweeney met.

Delia could only lie there, petrified, as he described how he’d shot Melissa in the flat they shared in Amsterdam, along with two German men he had found in her bed.

‘I shot them all,’ he told her. ‘Didn’t know what to do with the bodies so I cut them up and I threw them into the canal.’

Then, almost as suddenly as the assault began, Sweeney let her go.

When Delia finally summoned the courage to report his confession – and showed police his bizarre portfolio of artwork – she says she was ignored.

Delia tried to cut out Sweeney from her life, changing the locks, but one night he ambushed her outside her flat.

He forced his way in and launched yet another attack. This time he was interrupted by the arrival of police, called by a friend, who was concerned for Delia’s wellbeing because she had not turned up at an arranged meeting.

Chillingly, at the scene, police found a ‘killer’s kit bag’ containing a hacksaw blade, rubber gloves, lengths of rope, a large tarpaulin and lengths of duct tape and Sweeney was charged with false imprisonment and causing actual bodily harm.

But astonishingly, not least to terrified Delia, after a few days he was released on bail.

‘I told police, ‘He knows what time I finish work, he’ll be waiting for me’,’ says Delia. ‘On the 22nd of December, exactly what I said happened.’

It is no wonder Delia is angry. She thought she was going to die. She very nearly did.

‘I was halfway up the steps with the bike. It was the darkest day of the year, nobody was on the street. I kept looking around… and there he was.’

He brought the axe out. He bashed me in the side of the head. I held my arms up.’ One arm was fractured, while two tendons in the other one were torn.

‘He dropped the axe and he pulled out a knife. I thought I’ve got to do something, so I pushed [him away] and then he came up the step again, stabbed me through the breast, stabbed me in the thigh.’

Delia saw her own finger fly through the air, then saw Sweeney holding aloft the axe.

A compelling forthcoming ITV documentary about Delia and her attacker will be released at the beginning of November

‘I said, ‘I’m going to die now,” she says. ‘Then the neighbour came out. I heard another noise and then… he ran away.’

Sweeney then promptly disappeared, and 11 days later sent a letter to police at Scotland Yard bragging about his crimes, and boasting how they’d never catch him, describing the attack an ‘AXEident’.

For the next six years he was, to police at least, a ghost.

And then, on February 19, 2001, the remains of 31-year-old Paula Fields were found in a collection of holdalls, weighed down with bricks and tiles, in Regent’s Canal in London. Her head, hands and feet were missing.

Investigators, looking into Paula’s background, came up with the name of a boyfriend known as ‘Scouse Joe’ – it took little time for them to establish that this was Sweeney.

At the flat where he had been living, police found hundreds of pieces of his artwork, women with heads and limbs cut off, vile images. Sadistic. Some of it annotated with dates, and crude poetry.

Some of it was the very same artwork he had shown Delia.

Sweeney would be convicted of the attempted murder of Delia, but prosecutors deemed there was insufficient evidence to charge him with Paula’s murder.

It was as part of the Paula Fields enquiry that retired detective Sue Kendrick met Delia. Sue had the daunting task of supporting her through giving evidence in court, a process that was incredibly difficult given Delia’s understandable fury, not just at her own ordeal but the missed opportunities to act.

‘It’s completely understandable how let down she felt by the police and by the system,’ Sue tells the Mail.

On the day she appeared in court, a furious Delia very nearly walked away and it was Sue’s job to convince her to stay. The two women reached an agreement and Sue remains in no doubt that Delia’s resolve that day was ‘absolutely pivotal’ in getting Sweeney safely off the streets.

‘If she hadn’t gone back into court, the judge would have chucked it out and he [Sweeney] would have walked, and then he would have carried on killing. There’s isn’t a doubt in anybody’s mind about that.’

It wasn’t until the remains of Melissa Halstead (whose body had been found in a bag floating in a canal in Rotterdam, way back in 1990) were identified by a newly formed cold case investigation team in Holland, that the ‘eureka moment’ arrived.

It meant police finally were able to make link between Melissa and the murder of Paula Fields. Sweeney had been in a relationship with both women at the time they went missing.

With the time ticking down to the end of the 11-year minimum term Sweeney was serving for his attempt on Delia’s life, an international race began to build enough evidence to keep him behind bars.

In April 2011 Sweeney was found guilty of two counts of murder and sentenced to spend the rest of his life in prison.

The artwork that Delia had tried to bring to the attention of police, proved crucial.

Professor David Wilson can still vividly recall the day that a bundle of copies of Sweeney’s artwork was delivered to his university office, at the request of the senior investigating officer in the Sweeney case, as police in the UK and Europe wrestled with one question: were there more women?

This week he tells me: ‘It was quite clear that these drawings were narratives. They told us a story, John Sweeney’s story.

‘They were clearly biographical drawings and paintings, often with dates and place. These were not decorative drawings, these aren’t flower vases and sunsets. They were all sharp objects, axes, knives, women being tortured, tied up, decapitated, mutilated.

‘What came across quite clearly was an acute misogyny, sadism, a desire to dominate and to control those women with whom he was in a relationship or had briefly met.

‘He was clearly proud of what he’d done.’

Professor Wilson still remembers what he told police: ‘I said I’ve got no doubt he has killed more women.’

Delia was so traumatised by all that had gone before, she didn’t give evidence again.

But Professor Wilson is in no doubt that she is responsible for justice being served.

‘Delia Balmer, what a brave woman, she survived in the most extraordinary of circumstances.

‘[What she went through and heard] it’s the stuff of nightmares. All I can say is that I hope she isn’t having nightmares any longer but is taking some strength out of the fact that she has been the pivotal bar in the process that brought John Sweeney to justice.’

Until I Kill You: The Real Story comes out on 7th November at 9pm. The drama Until I Kill You is on ITV1 and ITVX from 3rd to 6th November at 9pm.