WHEN police finally arrested Ian Stewart for murdering his wife Diane, he shook his head and said: “You’re joking!”

       But the feigned shock of a calculating and narcissistic liar came as little surprise to officers as they read him his rights.

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       Killer Ian Stewart with fiancée Helen Bailey and her dachshund Boris in a pic from his Facebook page Credit: Enterprise 6

       Diane was found collapsed in the garden in 2010 Credit: PA

       For bearded Stewart, 61, had made the exact same remark when he was held for killing his fiancée Helen Bailey two years earlier. Now, however, the game for this sinister ham actor was well and truly up.

       After he had fooled the world for a decade that the mother of his two boys had died from an epileptic fit, this week a judge gave a whole life sentence to Stewart, whose motive for both killings was money.

       And the seeds of his second downfall had been sown when Diane had filled in a donor card to leave her brain to science. After her death, Stewart felt compelled to comply with it, but asked that no one — not even family members — should be told.

       But after his 2017 conviction for Helen’s killing, detectives began to look into his wife’s death in 2010, and asked scientists and pathologists to re-examine Diane’s stored brain.

       They found signs that her oxygen supply had been restricted — and this week Stewart’s fate was sealed when he was convicted of her murder.

       Yet questions remain as to why police, a coroner and pathologists all missed signs of foul play in the death of Diane, six years before Helen was murdered — particularly how Stewart managed to kill his wife, return to his mundane life, then murder his fiancée without being detected.

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       Trawling forums

       He and Diane had met as students at the University of Salford and got married near Diane’s home near Dronfield, Derbyshire in 1986.

       Software manager Stewart, a teacher’s son, had grown up in Letchworth Garden City in Hertfordshire. He and former school secretary Diane had two sons, Jamie and Oliver, who were 18 and 15 at the time of their mother’s death. Described as an “adoring” mother, she was a committee member of the local air cadets where her boys were members.

       It was a hot June day in 2010 when Stewart rang 999 to say 47-year-old Diane had collapsed in their garden at Bassingbourn, Cambs. She had suffered from mild epilepsy, but for 18 years had controlled the condition with medication.

       Yet when paramedics arrived, she was dead, apparently from an epileptic fit. After a coroner recorded a “sudden unexpected death in epilepsy” Stewart requested that his wife be cremated.

       It was a perfect way to destroy the evidence — except for the donor card. Diane’s sister Wendy Bellamy-Lee later said in evidence: “There was an element of suspicion because Ian had been on his own.”

       She said she told Stewart she had contacted the coroner to find out more about her sister’s death, adding: “He was really, really cross with me.”

       After a coroner recorded a 'sudden unexpected death in epilepsy' Stewart requested that his wife be cremated.

       Also, Stewart had given neighbours differing accounts of Diane’s collapse. He told one he had watched her through a window hanging out washing moments before she dropped to the ground. He told another he had “popped out for ten minutes” to Tesco and returned to find her collapsed.

       The 999 operator had told Stewart to perform CPR on his wife. But when paramedic Spencer North arrived, he found Diane’s body in the garden with no sign that Stewart had tried to perform CPR on his wife.

       The paramedic later told Stewart’s second murder trial: “From my professional experience, when you go to a death of a loved one or a family member, (people) typically show a heightened emotional state, screaming, crying, upset. In this particular case he appeared dissociative and was simply answering questions calmly.”

       At the scene, PC Matt Gardner did not believe the death was suspicious so he completed a coroner’s report form. He later testified about Stewart: “He answered my questions clearly, I wouldn’t say distressed, distraught, but people act very differently under such circumstances.”

       Experts later put Diane’s chances of suffering a fatal epileptic fit at one in 100,000. “Grieving” Stewart got on with his life, aided by the almost ￡100,000 he received after Diane’s death from a life insurance payout and her bank accounts.

       He bought an MG sports car and within months of her death he began trawling online bereavement forums for women. There he met millionaire children’s author Helen Bailey, 51, who was mourning the death of her husband John Sinfield.

       'Grieving' Stewart got on with his life, aided by the almost ￡100,000 he received after Diane’s death from a life insurance payout and her bank accounts.

       John, 65, her partner for 22 years, drowned in front of her while they holidayed in Barbados in 2011. Helen’s poignant writing would become a source of inspiration for others who had lost loved ones.

       And shortly before her husband’s funeral she began chatting to a “Gorgeous Grey Haired Widower” — Stewart — on a Facebook bereavement page. Diane had been dead for less than a year.

       At first their relationship was more akin to friendship. But romance eventually blossomed with the pair enjoying walking Helen’s beloved dachshund Boris on London’s Hampstead Heath, near her home.

       In 2013 the couple sold their homes and bought a rambling ￡1.5million house with an outdoor pool and an acre of land in Royston, Herts. Stewart and Diane’s sons Jamie and Oliver moved in with them.

       Helen — worth ￡3.3million — also changed her will to make Stewart the primary beneficiary. Her career flourished, and her 2015 memoir, When Bad Things Happen In Good Bikinis, was a hit. When it was serialised in a national newspaper and on Radio 4, a portly, greying Stewart stood grinning in the publicity shots.

       But her success may have galled him, as he had not worked for years. He suffered from a rare auto-immune condition, myasthenia gravis, which caused muscular weakness and breathing problems, though police later believed Stewart over-egged his illness to avoid having to get a job.

       The couple were due to marry in September 2016 but in April that year Helen vanished. At the time, Sun reporter Amy Jones investigated her disappearance but when she knocked on neighbours’ doors she was met with a wall of silence.

       Really agitated

       One said: “I’m sorry, I’d love to talk, but Ian doesn’t want us to speak to the Press.” Stewart claimed Helen had left a note saying she needed time alone and had gone to her holiday cottage in Broadstairs, Kent.

       Yet when The Sun’s Amy went to investigate, locals said they had not seen Helen for months, but they had seen Stewart.

       When asked if Stewart was concerned about Helen, a staff member in the local grocery shop said: “No, but he was very worried about what we’d said to the police.

       “He was asking us if we’d given them our CCTV footage and seemed really agitated when I said yes.”

       In July police swooped on Stewart but he was released on bail. Four days later Helen’s body, and that of her dachshund Boris, were discovered in a cess pit inside a garage at the rear of the house.

       It is thought Stewart slowly drugged then smothered her before dumping her body in the human waste. After a jury convicted him and he was sentenced to life imprisonment, attention began to turn back to Diane — could she have been murdered too?

       The examination of her brain suggested she had in fact been unconscious, slowly dying for an hour, with her breathing restricted. In 2018 Stewart was arrested — reprising his “You’re joking” routine.

       During his trial, prosecutors suggested he had sneaked up on his wife and either forced a plastic bag over her head or put her in a “sleeper” wrestling hold.

       This week at Huntingdon Crown Court Mr Justice Bryan sentenced Stewart to a full life term, saying he had deliberately carried out Diane’s murder when Jamie and Oliver were out of the house.

       How any father can act as you did defies comprehension.

       The judge

       The judge said: “You knew very well that your sons would return, and would return to such a sight. How any father can act as you did defies comprehension.”

       During Stewart’s four-week trial the two sons he professes to love watched from the public gallery as their father showed no contrition for his crimes.

       They made sure not to catch his eye. For the monster before them had not only smothered the author who was destined to be their stepmother, but their own mother too.

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       After his 2017 conviction for Helen’s killing, here team finds her body in cess pit, detectives began to look into his wife’s death in 2010 Credit: SWNS 6

       Scientists and pathologists re-examined Diane’s stored brain and found signs that her oxygen supply had been restricted, here Stewart bought an MG 6

       This week a judge gave a whole life sentence to Stewart, whose motive for both killings was money Credit: Not known, clear with picture desk 6

       During Stewart’s four-week trial, his two sons Jamie and Oliver watched from the public gallery as their father showed no contrition for his crimes Credit: sbna_fairleys