To the outside world, former Boyzone star Mark Walton is living the Hollywood dream.

Handsome and successful, the former singer-turned-record producer spends his days working alongside the likes of Lady Gaga and Jennifer Lopez, before returning home to a Beverly Hills mansion.

He has come a long way from the sleepy Dublin suburb of Donaghmede.

But for the past few months, another, darker world has started to intrude on Mark Walton's sun-drenched success, a world so disturbing that Mark describes it as a 'living nightmare', according to the Daily Mail.

Advertisement

It is a story of such monstrous paranoia and brutality that it culminated in the depraved murder of innocent French au pair Sophie Lionnet, tortured then killed by Sabrina Kouider and her boyfriend Ouissem Medouni. The case, which concluded last week, has gripped a public scarcely able to believe that the beating, starvation and even waterboarding they inflicted upon their victim could take place in leafy Wimbledon, or that Sophie's murderers could burn her body on a garden bonfire while barbecuing chicken close by.

Their victim was so badly charred that it proved impossible to establish exactly how she died.

Yet, for all the global interest in the case, few have followed it more closely than Mark Walton.

After all, glamorous Kouider was his former lover. And Mark himself has come to learn that, while he may have been more than 5,000 miles away from suburban South-West London and this real-life horror story, he had unwittingly played a central and disturbing role in the nauseating drama.

The two-month trial at the Old Bailey heard how Kouider was 'fixated' with Walton and was the driving force of what the judge described as 'folie a deux' – or a 'madness of two' – with Medouni the weak but willing accomplice.

They created a fantasy world casting Mark as an evil villain who had seduced au pair Sophie with sex and promises of Hollywood stardom. It was all a tissue of twisted lies.

Kouider and Medouni were convicted of murder last week and now await sentencing.

Mark's eyes are red from crying when we meet at his £500,000 recording studio in the heart of Hollywood.

At 42, he still has the boyish good looks that prompted impresario Louis Walsh to pluck him from obscurity to become a founder-member of Boyzone, the 1990s Irish boyband (Ronan Keating became its break-out star) who sold more than 25 million records with hits such as Words and You Needed Me.

Speaking in his soft Dublin brogue, Walton says he is 'haunted' by the final image of the tragic nanny, looking frail and emaciated, taken in his former lover's home.

'The whole nightmare was far away and somehow surreal until I saw that picture,' he says. 'There was a sense of incredulity. I'm in Los Angeles and I never met the poor girl.

'But then when I saw that picture of Sophie and how scared and vulnerable and haunted she looks, and to think she was tortured because of me, that haunts me. The poor girl was telling the truth. She never met me. And yet she paid with her life because of those psychopaths.'

Walton gave evidence at the trial, where it emerged that the two sadistic killers were for some reason obsessed with him, and that they had falsely claimed he was in league with 21-year-old Sophie, who they believed was spying on his behalf.

In fact, says Walton with tears in his eyes – a story he repeated to the judge and jury during the trial – he never met the timid nanny: 'I never met her, never spoke to her. There was no truth in any of it.

'Sophie had been beaten, battered and abused, had a broken sternum, broken ribs, fractured jawbone, and worse. And she was tortured all because of me, because Sabrina imagined that we had a relationship, though we never met.

'It's heartbreaking to me that she would suffer so much because of me. I would love to reach out to her parents. During the trial it didn't seem appropriate to do so but I want them to know their daughter is in my prayers.

'I think of her every day. I want them to know I carry their daughter's picture with me everywhere. I will never forget her. We never met but I will always pray for her and her parents.

'This case has shaken me to the core. I never drank, but now I need two glasses of wine to get to sleep. I have the worst nightmares.'

When Mark and Kouider's romance began seven years ago, it was like a scene out of a Hollywood romcom, one that might star Julia Roberts and Hugh Grant in – where else – Notting Hill.

Today, he asks himself how he could have been taken in. Yet he is not the first to fall for someone as glamorous as she was manipulative.

'I once loved Sabrina, wildly and passionately,' he admits. 'I was blinded by love, until I found out she was a calculating psychopath.

'We met in a NatWest Bank in 2011. She was speaking to the manager while I was in the queue.

'I asked him: "Who was that?" and he said: "She asked the same question about you" and introduced us.

'It was love at first sight. She was 5ft 8in and beautiful with a nice figure, big brown eyes, soft French accent, long brown hair, beautiful lips, gorgeous skin, and when she smiled I'd literally melt.'

Born in Algeria, 35-year-old Kouider grew up in Paris where, aged 18, she met Medouni, a fellow French Algerian five years her senior, at a fun fair. The pair lived together on and off for 17 years and even underwent a non-legal Muslim marriage but she continued to have multiple relationships with other men, including one which resulted in a son, now eight.

A sickening image shows nanny Sophie Lionnet after she was starved and tortured by the couple she worked for. Photo / Supplied
A sickening image shows nanny Sophie Lionnet after she was starved and tortured by the couple she worked for. Photo / Supplied

They moved to the UK in 2009 where Kouider worked as a nanny, make-up artist and would-be fashion designer. Medouni worked for a French bank.

'She asked me to meet one evening, which I thought was a date,' recalls Mark. 'But when I arrived there were 11 other men there, many of whom seemed to be under the same impression. It was a sales meeting for a pyramid scheme selling phone lines. Several guys left but I stayed because she intrigued me.

'I ended up buying into the company. She asked me to sign up, saying it was only £225. It turned out to be £800, plus another fee, plus monthly fees. That should have been a warning to me.

'But I paid because I wanted to help her. I'd been so focused on work I hadn't given much thought to women, but she changed that.'

At the time Mark was managing the boyband Blue while building a successful roster of production credits including Lady Gaga, Enrique Iglesias and Jennifer Lopez.

'It turned into a romantic relationship very quickly,' he continues. 'We started talking on the phone every day and saw each other every night.

'I was living in Notting Hill, but after five or six months I moved into her apartment in Queensway, paying her £2,000-a-month rent.

'I felt sorry for her,' he explains. 'She told me she'd been raped and abused. I thought I could save her.

'I was travelling the world and I would come home to her and there was a sweet, loving, gentle side.

'Sure there were problems and she would lie and have these rages but did I think she was capable of murder? Never.'

Yet there were signs of trouble early on. Kouider had violent mood swings, once hurling an ashtray at him and often berating her lover during screaming rows. Yet Mark says she wielded a psychological power over him, too.

'She said she'd been raped by two uncles. I wanted to look after her and make her feel better. I found her very vulnerable and, despite all she did, I believed I was going to make her better. I later learned it was all lies.'

The relationship grew ever more tempestuous as Kouider's mood swings became more extreme. 'She was emotionally and physically abusive. There were moments I felt in danger but I never thought she'd ever try to kill me,' he says.

Mark meets for this interview in a black Armani blazer, Hermes belt, charcoal grey jeans, weathered black leather boots and aviator sunglasses. He is the epitome of a successful music producer and is a household name in the Far East, where he is known as a judge on the Asian version of Pop Idol.

He still finds it incomprehensible that he was taken in by a woman he now calls a 'manipulative monster'. 'We'd been together 14 months and were living together at her apartment in Queensway when I was looking in a cupboard for my notebook and found some photos showing her with another man.

'I asked her who he was and she exploded: "Why are you rooting around in the cupboard? You're trying to control me!"

'She threw 30 photos out our fifth-floor window, they went floating down to the street. I turned to leave and she hurled a heavy glass ashtray at me, which shattered against the wall.

'There were punches and slaps. She once woke me with a punch to the jaw because I was snoring. I was bruised for days. We had four full-time nannies for her older son [now eight who cannot be named for legal reasons] and she accused all of them of sleeping with me.

'She set up hidden cameras in the house to spy on them. She was convinced it was true, though there was never any truth to it.

'It was one of the scariest things when this beautiful girl would turn into a psychopath.'

Why did he stay in such an abusive relationship? 'It's crazy, I know, but I was in love with her,' he admits. 'With all that she's been accused of it seems like she's a crazy, sex-mad girl. But we had the most average, normal sex life. We didn't have sex that often.'

But after 14 months together Kouider became pregnant. 'I was the happiest guy in the world, ecstatic. She led me to believe that I was the child's father.

The victim is apparently Sophie Lionnet, 21, who was caring for her alleged killer's children. Sophie has not been seen for a couple of weeks.
The victim is apparently Sophie Lionnet, 21, who was caring for her alleged killer's children. Sophie has not been seen for a couple of weeks.

'After a few months she disappeared back to France saying her mother was ill. A few weeks later she called to say: "The baby died, I had a miscarriage. Get on with your life." Then she hung up. I was devastated, but when I talked to her brother he said it wasn't true. The baby was alive.'

The child is now four and believed to be in the care of Kouider's family, though Mark is hoping for a DNA test and wants to be a full and supportive father if the child proves to be his.

Despite Kouider's abuse, Mark stayed with her for two years, until she left him a few days after he moved to LA and, eventually, cut off contact. He continued sending her thousands of dollars while hiring a succession of lawyers to try to get her to agree to a paternity test. 'For so long I've considered the baby my son, but I honestly in my heart don't know. Finally I stopped sending money'

It was when Mark cut off financial support that she turned on him. 'She contacted some of my biggest clients – major international music stars – telling them I was a paedophile. That's been very hard for me to get over. I can't think of a worse accusation.

'I only learned during the Old Bailey trial that she had called the police 30 times about me.

'She told police I was sexually abusing her cat! But we didn't have a goldfish, let alone a cat.'

When he was interviewed by police, Mark was stunned to learn the extent of his ex's depraved lies including claims he had 'snuck' into her flat to extract semen from her sleeping boyfriend.

'It was like one of the worst horror movies you can imagine.'

When he saw Kouider in the Old Bailey dock, he says: 'I felt sick to my stomach. For the first time, I felt hatred towards her.

'But to have this fixation on me and for that poor girl to lose her life because of it? I thought: "How could you do such an evil act?" What Sabrina did was beyond evil, depraved and vile.'