So why didn't he call 999 if he'd done it? Can anything really be read into this?
Certainly it can. It goes a bit like this. When one first gets the script for a play there's a cold read-through, ie we just read the written words. At that time we're not 'into' the character we're hoping to play. By the time the play is staged, we're living the character we're playing. We feel what they're feeling. It seems to me that Jeremy was at the read through stage. Although it was technically correct, it lacked feeling, and the reason it lacked feeling was, because there was no phone call for him to draw on, he hadn't experienced the terror Nevill was likely to have felt HAD he felt compelled to call his son at that hour, and only remembered, at the last minute, when complaining about being kept waiting, that it might be a good thing to say his father sounded terrified.