The prosecution can just refer to the evidence regarding the time window when Nevill had to call Jeremy - after shots 1-4 & before the kitchen attack.
The prosecution could not say -
Sheila would have chased Nevill so he had no time to phone.
Nevill would not go downstairs once he sees Sheila is the shooter.
Nevill would not phone Jeremy after getting downstairs.
Nevill would not use the slow rotary phone.
If Sheila shot the twins before going downstairs, Nevill would still not have time to phone Jeremy.
Nevill would phone 999 for the police and ambulance instead.
Jeremy would have his AM on so would be of no use.
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These things are subjective & the defence would just say the opposite.
So the prosecution just said there was no blood on the phone & Nevill would have difficulty speaking. Which is evidence based.