Double-murderer Ian Stewart would have escaped justice if it hadn't been for his slain wife's decision to donate her brain to science.
The twisted dad-of-two will spend the rest of his life behind bars after being given a whole-life order today.
The 61-year-old had already been jailed for the murder of his fiancée children's author Helen Bailey at their home in 2016.
But six years earlier his wife Diane Stewart, 47, had also died - though it had been put down to ‘sudden unexplained death through epilepsy’.
Stewart had claimed he found her collapsed on the patio of their home in Bassingbourn, Cambridgeshire, after returning from the supermarket.
However, after he was convicted of killing Ms Bailey, 51, the previous death was re-examined.
Mrs Stewart had been cremated but by a stroke of fortune she had had her brain donated for medical research.
When pathologists re-examined it - crucial evidence, missed in the original post mortem, revealed there was only a one in 100,000 chance that she would have died from epilepsy.
Her death was deemed to have been carried out either by use of a plastic bag over the head, or by a choke hold.
A jury of seven women and five men at Huntingdon Crown Court heard how it had been 18 years since Mrs Stewart had last had a seizure.
They also heard he made no effort to carry out CPR on Mrs Stewart, with judge Mr Justice Simon Bryan concluding it was likely she was already dead when the killer made the 999 call.
A paramedic gave damning evidence of how there was nothing to suggest Stewart tried to resuscitate his wife.
The judge slammed the defendant for a "concerted and callous charade".
Years later, Stewart spent weeks poisoning new partner Helen with prescription sedatives.
As she excitedly planned their wedding with the man she referred to in her writing as "the Gorgeous, Grey-Haired Widower", she is believed to have fallen asleep at her computer, overcome by the power of the pills.
He then smothered her in a “heinous” plot motivated by greed and dumped her body in a cesspit under their £1.5m home alongside her dog Boris.
Their bodies lay undiscovered for three months.
Judge Andrew Bright handed Stewart a life prison sentence in 2017, with a minimum term of 34 years before he could be considered for parole.
Helen had been worth around £4million at the time of her death.
Jurors in the second trial heard Stewart received a total of £96,607.37 following the death of his wife, from her bank accounts and including £28,500.21 from a life insurance policy.
Judge Mr Justice Bryan said there were striking similarities between the two murders.
"It no doubt never crossed your mind that the donation of Diane’s brain for teaching and research would lead to your ultimate downfall, as it was to do, and your conviction today for the murder of Diane Stewart, for which I must now sentence you," he said.
"I, like the jury, am sure that you killed your wife Diane. I am also sure that there are striking similarities between your murder of Helen Bailey and your murder of Diane Stewart.
"So far as Helen Bailey this was a murder with a significant degree of planning and pre-mediation in relation to a murder for financial gain, you told a series of calculated and callous lies over an extended period of time, and you acted out a charade in an attempt to deceive everyone that she had simply gone away, and that you were not involved in her death.
"You murdered Helen Bailey a lady with whom witnesses in that trial, and in this trial, said you were very much in love with and indeed about to marry just as you murdered Diane whom you also said were you very much in love, as again attested by many witnesses."
The month-long murder trial heard how Stewart had splashed out on an MGB sports car after the murder.
Stunned friends reported being surprised at his behaviour during his wife’s funeral, appearing to be “totally unbothered” and “seemed quite aloof”.
Within a year of the murder he had met children's author Ms Bailey, who penned the popular Electra Brown books.
The besotted couple were seen walking around Stewart’s home village, with the killer appearing to have a new lease of life, according to witnesses.
And Mrs Stewart’s sister, Wendy Bellamy Lee, revealed how the killer had fumed at her after she asked the original coroner for more details about the death of her sister.
In a heartbreaking tribute paid today, Wendy and brother Christopher Lem, said: "Tragically she died far too soon, she will always be in our hearts."
