Sophie Lionnet was tortured and murdered by the pair (Scotland Yard/PA)

A delusional couple have been jailed for at least 30 years for torturing their French nanny to death over a bizarre obsession with a Boyzone pop star.

"Vindictive and overbearing" Sabrina Kouider (35) and her "timid" partner Ouissem Medouni (40) killed 21-year-old Sophie Lionnet over a false belief she was in league with music mogul Mark Walton.

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Whatsapp Sabrina Kouider had constructed a bizarre fantasy (Scotland Yard/PA)

Afterwards, they threw her body on a bonfire in the garden of their Wimbledon home in September last year.

Kouider went on to try to frame Mr Walton for her disappearance to exact "revenge" on the former boyband member.

Following an Old Bailey trial, the defendants were found guilty of murder, having admitted perverting the course of justice.

Jailing the pair for life, Judge Nicholas Hilliard QC said: "It is plain from all the evidence that Sophie was a kind, gentle and good natured girl."

He said the allegations she was plotting with Mr Walton to abuse the family were a "complete fiction".

There was no excuse of the "horrible cruelty" and "humiliation" the defendants exacted on Miss Lionnet during taped interrogations before her death, the judge said.

He said: "I'm sure on all the evidence you were both involved in torturing Sophie in the bath in the lead up to her death in making her think she would drown unless you gave her information you wanted which was not in her power to give because it did not exist.

"The suffering and the torture you put her through before her death was prolonged and without pity."

He said their behaviour had its "origins in cruelty and a desire for revenge".

He told Kouider she was motivated by an unjustified desire to make Ms Lionnet and Mr Walton "suffer", despite suffering from a mental disorder.

He told her: "I do not think you thought for one moment you were acting lawfully. I'm sure you knew the way you interrogated her was unacceptable in the extreme, that it was unlawful to assault her and she was in a dreadful state by the time of her death and torturing her in the bath was totally and utterly wrong."

The court heard Kouider's delusional disorder triggered her false belief Miss Lionnet had been seduced into helping Mr Walton.

Over more than eight hours of recorded interrogations, the au pair was slapped, likened to a Nazi collaborator and called "worse than a murderer" by the couple.

They confiscated her identity card and phone and stopped paying her in a deliberate campaign to isolate her and force her to confess, the court heard.

In her final days, Miss Lionnet was hit with an electrical cable and beaten so badly she had five fractured ribs and a cracked breast bone.

She appeared broken and emaciated in a videoed confession hours before she was killed in the bath.

The defendants claimed they planned to hand the tape to police as proof against Mr Walton.

But the judge suggested it was more likely they would have sent it to the wealthy music mogul to force him to give them money.

When firefighters investigated pungent-smelling smoke coming from a bonfire, Medouni tried to pass off her charred remains as a sheep.

The defendants later admitted disposing of Miss Lionnet's body but denied her murder, blaming each other for her death.

Following their convictions, Miss Lionnet's tearful mother, Catherine Devallonne, said: "These self-obsessed individuals who murdered Sophie did not believe Sophie had a value.

"Those monsters repeatedly beat Sophie. They starved, tortured and broke her until she could no longer fight.

"They took away her dignity and finally her life painfully ebbed away until Sophie struggled to take her final terrified breath in the bath."

The victim's father, Patrick Lionnet, said what the couple did to his shy and reserved daughter was "beyond comprehension" and "unforgivable".

In a letter addressed "dear Sophie", Kouider acknowledged the suffering of her victim and her family, saying how "deeply sorry I am about what happened to Sophie".

She wrote they shared some "good times" until "things went terribly wrong".

"I am shocked and sad you are not part of this world anymore.

"I'm suffering every day thinking of you and what happened to you that dreadful night. I only wish I could turn the clock back, it never happened and you would be alive with us today," Kouider wrote.

In mitigation, Icah Peart QC said the fashion designer's delusional disorder coupled with an emotionally unstable personality disorder had caused her "irrational and completely overwhelming fear" that Miss Lionnet had been recruited by Mr Walton.

He said her behaviour towards the victim was "entirely driven by Ms Kouider's mental illness in her desperation to obtain evidence of Mr Walton's abuse".

By the time of Miss Lionnet's death, it was Medouni who had taken over the "dominant role" in her interrogations, the lawyer argued.

But Orlando Pownall QC, said Kouider was the "dominant" party who preyed on the "weak", including his client Medouni.

He said the former banker had been "indoctrinated" by a woman with the capacity for "sudden and extreme violence".

"Such was the all consuming nature of his commitment to Sabrina he did not have that many friends at all."

He said Medouni's love and desire to protect became "warped" and his inactivity and tacit support led to him being complicit in Kouider's scheme.

He added: "But for Ms Kouider's obsession, Miss Lionnet would still be alive."

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