Operation eliminate the Cuckoo
Step one: Frame him for murder.
"We discussed the implication of how this silencer could be in the gun cupboard with blood and paint on it. Obviously if it was being alleged that somebody had had a brainstorm and shot dead four people they would surely not have stopped to remove the silencer, put it back in the gun cupboard, go back upstairs and shoot herself dead. Contact was made with the police about the discovery of the blood and paint stained silencer."
Step two: Swindle the remaining inheritance (possibly by implying he died in the shootings)
"Ann concedes that the conversations regarding Mabel’s will were conducted partly ‘to stop Jeremy benefitting through aunt June’ but the family were unanimous that Jeremy showed no interest in calling on his grandmother.
Two days after Mabel was told about the murders, Robert asked her solicitor to call at Vaulty. Six persons were present: Mabel, Robert, Pamela, Dr Ellis and Mr Peek and his secretary. Robert recalled that the meeting ‘resulted in a solicitor from this firm and myself being made joint executors to her estate. The new form of the will was that Pam was made the sole beneficiary.’ Why Jeremy’s name didn’t come up at the meeting, and particularly why Mabel apparently failed to ask about him at this point (given that Ann recollected she made no mention of Jeremy until December), is unclear."
Step three: Get Ainsley on the case because Taff Jones had a brain.
"The 1986 internal review affirmed that ‘it was known to the Boutflours that had Jeremy inherited the estate he intended to sell what he could, thereby disposing of what had been part of the Speakman family estate. In addition to this, he would also have sold an area of land which [Nevill] Bamber had purchased intending to sell it at a later date to Peter and Ann Eaton when they had sufficient funds.’ The review noted that while it was not suggested that this interest had in any way influenced the Boutflours in their statements to the police, it was known to DCI Jones during the initial stages ‘and may have been a factor which affected the level of credence he placed upon the information given by the relatives’".
Step Four? It would not suprise me if once they heard Julie and Jeremy broke up they took the first opportunity to con her into believing what they wanted people to believe also.