It was Pc Holby Smith, not Holtby Smith. Saying a clock is accurate ± 5 minutes isn't a way of saying that it was out by 5 minutes. It is a way of saying that the clock was unreliable and, from memory, was sometime fast or slow by as much as about 5 minutes. This assertion was clearly rubbish anyway, as such a clock would have been reported and fixed, not tolerated (and that applies regardless of which clock Pc Holby Smith was referring to). Also, Pc West gave a different (and far more believable) account of the clock's accuracy. He clearly believed the clock was correct and that his recorded time of Jeremy's call was correct as well.
My understanding is that Jeremy has never confirmed trying to call Witham police station, but not getting an answer. I think he has denied doing so.
We can't be certain about when CA5 was sent, but its driver, Pc Cracknell, made a statement (available on this forum) in which he said that he and Pc Norcup (who together were the crew of CA5) were on duty in the Chelsmford police station control room (the room where Pc West was working) at 3:33 am when they were requested to attend White House Farm. There is nothing known to explain why they took about 48 minutes to drive to WHF. Once on their way, they would have confirmed their departure by radio, allowing Bonnett to update his log accordingly.
The kind of conversation suggested by Caroline (even with the suggested additions) would easily fit into two minutes, which leaves Jeremy just enough time to arrive at WHF at the time suggested by the police. He was estimated to be doing about 30 mph when overtaken by the police, which is consistent with the times I have mentioned. It's clear that Jeremy had called Julie before he called Pc West.