Yes I think it's credible, he still does it today. Why did he need a precedent? The police fell for it at first, and people still do today.
Since Ralph called the police from whf, and told them that his daughter had got one of his guns (3:26am) and that she was going berserk, Ralph must have been in on the plan to use Sheila's mental health issues as well? Jeremy never told Malcolm Bonnet that he was Sheila's dad, and that she was his daughter, or that Sheila had got one of his dads guns, and so Bonnet could not relay that information to PC West for him to make the mistake of writing it down wrongly. In any event there is no evidence from Bonnet saying that he told PC West exactly what PC West recorded at 3:26am, except at the bottom of the message where West makes mention that the son contacted the police and reported a telephone call from father, etc...
The 3:26am telephone log consists of two separate messages, first part from Ralph, second part from Bonnet reporting what Jeremy had told him, whereas, the other telephone log times at 3:36am, is all about what Jeremy told the police when he made his call to the police...