Had he been woken, and was out of bed but still in the bedroom maybe?
It's strange that the shell casings are all in that corner of the room.
If Jeremy did it, yes, I think Nevill had got out of bed just before he was shot. If Sheila did it, I think he had been downstairs to ring Jeremy, and then came back upstairs into the bedroom and was shot four times by Sheila.
I haven't even had much chance to look at the casings yet. I did so years ago, but I've forgotten their position now.