Isn't this what JB was more or less claiming? For some reason Julie pointed the finger at MM but maybe even they didn't know.
MM is said to have a solid alibi so obviously they picked the wrong person.
Is there anyone else who thinks there was a third person?
I've given the fact re. a third person some consideration over the years , though when you think about it, because the family concerned were taken off-guard, there was no preparation in place when confronted by someone brandishing a rifle as " they're in charge " and you don't argue. It only takes a couple of shots to put them out of action.
Probably the truth of the matter is that Nevill didn't think for one minute that he'd lose control of his daughter, though in the past he has rang for JB's assistance when Sheila had been difficult. This time was different because Sheila had a gun which would very likely have scared JB just thinking about it.
I could be right in thinking that when Sheila had her melt-downs in the past that she wasn't brandishing a gun, although even if she had JB wouldn't have said so for fear of being disbelieved.