The more I think about it, the less plausible Holly's scenario becomes.
Without exception, every Bamber supporter is always forced to take the least likely explanation.
However, if Bamber didn't do it, there must be a plausible way that SC did.. so here is my jackanory version.. I'm afraid I have left a few holes.
Sheila has gone to bed around 10-11 ( I think ) - but she gets up in the night, goes downstairs and starts crashing around or talking to herself waking NB who either gets out of bed and goes downstairs or is already there because he fell asleep in his chair.
Let's say she already has the gun and put the cartridge in before NB wakes up. She doesn't find and put the moderator on.( it's definitely a frame) He confronts her and she is holding the gun under her chin. She isn't making any sense and he can't get through to her. She isn't making too much noise and waking up June who is still in bed so it's a muffled kind of "going crazy" - not the shouty hysterical kind of crazy she had been prone to. He can't get too close because he is afraid she will pull the trigger and she backs away and barricades herself in the lounge or something.
"Hmm" thinks Nevill, "I need some help here, I can't wake June cos she needs her beauty sleep..if only there was some kind of emergency service to contact in this situation... there's only one hero that can save the day, Sheila is always happy when she sees Jem!" He picks up the phone. He has just enough time to blurt out the immortal words when he realises that Sheila is already halfway up the stairs so he cuts the call, thoughtfully leaving the line clear for super Jem to be able to ring Julie but leaves the handset on the worktop and races upstairs to tell Sheila the good news that her favourite brother is coming round. He doesn't make it to the bedroom before he gets shot a couple of times and turns around and gets shot twice more as he races back down to the phone to ring Jeremy back and tell him to bring some elastoplasts, but he isn't fast enough and he is too injured to put up much of a fight.
After battering the father she loves and always turned to when she was having an episode, while skillfully avoiding the broken glass and sugar on the floor, she calmly reloads, finishes him off and then goes upstairs to finish off June and the kids she adores. Then she goes back downstairs to reload again,decides to lift NB onto the coal scuttle, smokes a cigar and burns NBs back with it before wandering back upstairs,where she fires a few more rounds just to make sure they are all dead and after having a good wash and touching up her nail polish, shoots herself with the last two bullets.
Meanwhile, in Goldhanger, Jeremy phones Julie for a quick chat, forgetting to mention that his father had called, it wasn't a big deal, His father was always phoning in the small hours saying Sheila had gone crazy with a gun.. he didn't sound terrified or anything..
My point is, you can just about make a plausible scenario where SC has a gun and NB is downstairs but it starts to fall down at the point NB is supposed to phone JB because in order for that to be remotely plausible, Sheila cannot have fired any shots at anyone and neither can she have threatened to shoot anyone else. NB would have had to have some reason to think it safe to turn his back on her. I also question Holly's version that he drops the phone upon hearing shots upstairs because it changes the situation in which he made the call after he has said the first 7 words. Why not "Sheila's gone crazy, she's got the gun..(bang)...........Help, she's upstairs call the police"