I see what you mean Rochford-Jeremy does indeed make a big "jump" back
in time within this statement.
Could it just be a case of "Oh, I forgot to mention the first 'phone-call to Julie "
after he tells of the second much later one to her?
It seems that in his statement his main focus of attention was on his Father's call to him and then his own call to the Police and going to the Farm and meeting up with the Police.
Perhaps that was so traumatic to him that his first call to Julie seemed irrelevant.
But it also begs the question of why he made the first 'phone call to Julie at all during such a time!
It is an aspect of his behaviour that I have never understood - why call Julie at 3ish, in the morning, all she tells him is to "go back to sleep" anyway!
That first call was an irrelevance during a crisis that I just don't understand unless he thought he might be injured whilst at the Farm.
Or unless he wanted to make yet another call from his Cottage that would firmly place him in Goldhanger
at that particullar time? 
Phone call to JM.
If JB was the killer the call to the police would be enough to place him at his home at the time it was made. There would be no need to make a call to JM.
If JB was the killer a call to JM would more likely draw suspicion on him ...though it could be argued he used it as a double bluff. Far better would have been no call to JM if he had a desire to hide the truth of what he had done.
On balance it does not suggest guilt at all in my view.
If innocent and Ralph made the call as JB claims...then the act of Ralph calling him and not the police means that Ralph did not want to involve the police. This argument holds with being credible.
If the call had not been ended abruptly as JB claimed then its likely JB could have gone to whf to assist his father in dealing with Sheila. This muddied the waters as something had happened to end the call which JB would be unaware of.
So a dilemma is created in which JB will be unsure what to do and what to make of the call he had. Does he involve the police or deal with it himself which is probably what his father wanted and was the purpose of the call. A difficult decision in my view.
Now if the call to JM was before the police were called it is quite credible that in wondering what to do he wanted to seek advice from JM about what was the best course of action. Seemingly JM was not interested in even listening so was of no help whatsoever.
If the call to JM was after the police then JB for comfort/re-assurance may have phoned her as he seemed nervous(going by his slow drive to whf), again she seemed to show no interest and seemed to just want to get her sleep rather than be bothered with something going on in the early hours of the morning that might be nothing at all or some silly fasle alarm.
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Why the jump in the statement?
We are not sure if some of the statement is missing or not...the bit about a person being observed in the bedroom would fit in.
Also we need to consider the person taking the statement leads the statement and thus things are in their control and jumping around in the statement is often their doing.
An additional element that affects the first statement considerably is the presence of others when the statement was taken. Even more so when some of them (according to their own statements) have butted in and interrupted things or asked questions of either the police officer concerned or of JB.
Thus this statement cannot be relied on as much as some could as it was taken in rather adverse conditions that were far from satisfactory. The presence of others, especially persons known to the witness, can affect what the witness will say and stops them being fully open or may inhibit answers or influence answers.