A French couple who tortured their nanny to death before burning her body on a bonfire in a bid to get away with murder have both been jailed for at least 30 years.

Sabrina Kouider, 35, and Ouissem Medouni, 40, subjected 21-year-old Sophie Lionnet to weeks of punishing interrogations to extract a false confession that she had betrayed them.

The couple were warped by Kouider's obsession that her ex-boyfriend, Boyzone founder Mark Walton, had been working with Sophie to spy on their family.

In heartbreaking recordings and videos, Kouider and Medouni threatened to have Sophie thrown in prison if she did not confess to her supposed betrayal.

In her final days, Sophie was filmed looking gaunt, traumatised, and "broken" as she offered the 'confession' she believed would convince the couple to let her go.

She had been beaten with electric cable by Kouider, and repeatedly held under water in the bath by Medouni. It is believed she was either beaten to death or drowned at the couple's Wimbledon home in September last year.

Medouni, a former financial analyst, was caught red-handed burning the young French woman's remains on a bonfire in the garden, while barbecuing pieces of chicken nearby in a bid to cover the smell.

At a trial at the Old Bailey, Medouni and aspiring fashion designer Kouider blamed each other for Sophie's death but were both convicted by the jury of murder.

Sentencing them to life in prison with minimum terms of 30 years, Judge Nicholas Hilliard QC, the Recorder of London, said: "The suffering and terror you put her through before her death was prolonged and without pity.

"With conduct such as this, in all the circumstances of what happened to Sophie, I don't think it matters who was the principle and who was the secondary party.

"Deliberate and sustained conduct like that has its origins in cruelty and a desire for revenge, which means the delusions have much less significance."

The judge said the couple "knew what you were doing was very wrong", telling them: "You reduced Sophie to a dreadful physical state where her spirit was entirely broken, and that's an awful thing to do to another human being."

At court this morning, a letter Kouider had written to Sophie herself was read out, saying she was "deeply sorry about what happened".

Kouider said they had shared good times "until things went terribly wrong and ended up in this horrendous tragedy".

"I wish I could turn the clock back and it never happened and you would still be alive with us today", she wrote. "Sophie, I wish things had been different, I hope you get some peace with God".

Kouider's obsession with Mark Walton, now a successful LA music producer, began after their two year relationship ended in 2013, as she blamed him for problems in her own life.

She made wild accusations that he had used black magic against her, sexually abused Medouni, and drugged them both - claims the judge said were false.

Kouider, now diagnosed as suffering from a personality disorder, deliberately isolated Sophie as she enveloped her in the bizarre claims against Mr Walton.

Sophie's mother Catherine Devallonne, who was in court alongside father Patrick Lionnet today, had begged Kouider to free her "fragile and beautiful" daughter in the weeks before the murder.

However, Kouider, obsessed with the idea that Sophie was "in league" with Mr Walton, held the helpless young woman prisoner for at least eight days before her death.

Angrily trying to extract information from Sophie, she and Medouni likened her to a Nazi, suggested she was "worse than a murderer", and suggested she was facing up to 40 years in prison.

They murdered Sophie in the early hours of September 19 last year, and kept the body in a suitcase for nearly two days.

They initially settled on burning the body in the garden, but the plan was thwarted when a concerned neighbour called the fire brigade and firefighters discovered the charred remains.

Orlando Pownall QC, for Medouni, told the court Kouider "played on the weak" and was "physically and mentally dominant" in the relationship.

"She indoctrinated him as she others", he said. "Initially he wanted to move on and not be besotted by the allegations made - by Ms Kouider was unwilling or unable to do so.

"It is plain that Mr Medouni was, in relation to her, weak."

However, Icah Peart QC, for Kouider, argued her actions have to be mitigated by her obvious mental health problems.

One of Kouider's former boyfriends, Anthony Francois, told the court she had a “dual personality” and would attack other woman in the street at random.

“She could be as loveable as she could be detestable, with no explanation for the changing of her moods,” he said.

Kouider and Medouni, both from Wimbledon, admitted perverting the course of justice and denied murder.

They were both found guilty of the murder charge after a trial.

