Wishart argues in his book that Johnston may have killed the Crewes, as well as fabricating evidence to frame Thomas for the crime.
He claims that Johnston was nicknamed "The Fitter" and had a reputation for threats and violence.
Wishart - who admits he has no evidence to support his theory - suggests that Johnston, who investigated a reported burglary at the Crewe's house in 1967, may have realised it was an insurance job, instigated by Harvey.
"Perhaps taking a shine to Jeannette, he confides at some point that he knows Harvey did it to claim the insurance, and that if Jeannette doesn't do him some favours or pay him some money he'll arrange for Harvey to be arrested and maybe even her also.
"Jeannette caves in to avoid the scandal, leading to her increasing fear of being in the house alone."