A founding member of the pop group Boyzone who has become entangled in a murder case involving his ex-girlfriend’s nanny has told the Old Bailey that he never met the victim.

Mark Walton appeared in court on Monday and told jurors that his former partner, Sabrina Kouider, 35, would go crazy over trivial problems and “flip” during their turbulent two-year relationship.

Kouider and her partner Ouissem Medouni, 40, are accused of murdering Sophie Lionnet, 21, at their Wimbledon home and then burning her body on a bonfire whilst cooking chicken to mask the smell in September last year.

Jurors heard that obsessions about Walton lay at the heart of Kouider’s campaign of torture against Lionnet. Kouider, a former fashion designer, thought Walton was controlling the au pair “using black magic”.

Based in Los Angeles, Walton told the court on Monday he had never heard of Lionnet before murder detectives contacted him.

Walton told jurors he co-founded Boyzone in 1993 and was in the band for about a year before going on to be involved in Fifth Avenue.

The Irishman said they had met in a NatWest bank in Notting Hill and hit it off straight away. He said: “I was in love. She was my life then.”

Walton said he would support fashion designer Kouider with thousands of pounds every month, even paying her rent long after they separated.

Walton said he paid for Kouider’s nannies, adding that she would often fire them over accusations of stealing and of being interested in him, the Press Association reported.

He said he challenged his then-partner about her behaviour, adding: “I think some of them were friendly. I’m a friendly guy, but there was nothing more than that.”

The musician said they lived together for two years in Queensway, London, before she disappeared. He tried to rescue the relationship by paying for her to fly to LA, he added.

On their relationship, he said: “Sabrina shared some stories from her past.

“I guess knowing that, I felt it brought us closer together at times, but it was turbulent, probably the most turbulent relationship I had ever been in.

“She would go from softly spoken French accent then she would flip, get very angry, very loud and just not care where we were.

“She would just go crazy over something trivial.”

The court heard that in 2013, he paid £12,800 in rent on Kouider’s Wimbledon flat but stopped financially supporting her in February 2014.

The next time he heard of Kouider was when she accused him of being a paedophile. Walton told the court that his former partner rang his mother in Dublin and made accusations, while also contacting his business partners and creating a fake Facebook page.

Referring to outlandish accusations levelled at Walton by the accused, prosecutor Richard Horwell said: “Have you ever been party to a plot to drug the people in the Wimbledon flat and, whilst unconscious, sexually abuse the occupants?”

Walton said: “Absolutely not.”

Kouider and Medouni have admitted perverting the course of justice but deny murder.

The court heard that in 2012 police had been called a number of times over various accusations from Kouider, including that he had photos of another woman on his phone.

In July 2014, she took out a non-molestation order, accusing Walton of harassing her.

The Old Bailey has heard the couple tortured Lionnet for hours in the bath and forced a videotaped “confession” which they planned to use to embarrass Walton.

More than eight hours of mobile phone recordings were played to the jury.

The trial continues.