Once a credible explanation of what Nevill was doing has been made, an explanation of how Nevill found the time to ring Jeremy needs to be suggested.
If Sheila was enraged enough to wake, go downstairs bare footed/in a nightie & then load, seeing Nevill would just make her start shooting. So no time for phone calls.
If Sheila was holding the rifle but calm, Nevill would reclaim the rifle and lock it away. So no need for phone calls.