When you consider who is guilty of a murder there are three things to consider. Means, motive and opportunity. Who had all of these things regarding the White House Farm murders? Sheila? If you take the view that she was having a psychotic episode then that takes the motive part out of it. Opportunity, well she was in the house. Means? She simply couldn't have fired the rifle so accurately and failed to hit the target only a few times, she couldn't have reloaded the rifle at least twice. She couldn't have battered Nevill senseless. She went on a killing spree including a life or death struggle with Nevill and showed little or no signs of this at all on her skin, hair, clothing etc. If this was to have worked she would need a steady supply of ammo. As far as I understand it she would have gone on the rampage in a flimsy nightie with no pockets. Finally she somehow managed to shoot herself through the neck twice, despite the fact that there is every indication that the first shot would probably have rendered her unconscious, and possibly even paralysed. Dr Vanezis's comment that the first shot wouldn't have been immediately fatal has been taken on the blue forum to mean she could have walked around and even run upstairs. This simply could not have happened. Despite what it said in the TV programme it's reasonably clear that Nevill was first shot upstairs. The fact he ended up downstairs was probably because he was trying to get to the phone, which was disabled either because the phone had been moved from his room without him knowing or by taking the kitchen phone off the hook. Someone battered him senseless because he was trying to get to the phone. What would it have mattered to Sheila if he had made a call for help? When she was planning to kill herself anyway which she would have easily accomplished by the time the police would have arrived? So someone was willing to go to great lengths to stop Nevill getting to the phone to say what was really happening. And who would have had the motivation to do that?
Bamber on the other hand had motive (money), means (knew how to get to the farm quickly on a bike and how to get into the building without having to break in) and opportunity (at last everyone in his way of his inheritance just happened to be gathered together in the house at the same time). It seems quite obvious really when you think about it, doesn't it?