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.

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

Following an Old Bailey trial, the defendants were found guilty of murder, having admitted to 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."

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.

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".