If MM did it, Jeremy would not be daft enough to ring the police.
He would stay in London with Julie and claim he was taking authorised annual leave. Being nowhere near the scene when the bodies were found or when the massacre could have happened.
Julie also said Jeremy did not have £2,000 to pay.
Jeremy would not hire MM. He was not a hit man for hire, or some sort of tough guy. To expect him to suddenly massacre a family is unrealistic. There is every chance he would back down at the last minute or during the massacre. Jeremy wouldn't.