Assuming Nevill was under duress of some sort why would he telephone Jeremy?
The theory is that he was burnt 3 times to force him to phone Jeremy.
This was to lure Jeremy to the farm.
It almost worked - except Jeremy phoned the Police.
Presumably the torturer wanted all the family dead including Jeremy.
Whoever this person was knew there was 2 possibilities.
1) Jeremy would turn up alone and would therefore be killed
2) Jeremy would turn up with the Police and the killer would leave the farm under the cover of darkness.
possibility 2) fits with WPC Jeaves seeing a rifle in the window
possibility 2) fits with the possible sighting of movement in the window
possibility 2) fits with the Police reporting a hunched up man seen leaving shortly after their arrival
If this theory is true then it is highly likely that this person
a) threatened Julie to make her confession
b) was somehow involved in the planting of the silencer
As a theory it fits a lot of known evidence ...
ANYBODY that somehow benefited from Neville and June's estate would have to be on the suspect list if this theory is true.
If it is true then the inheritance would presumably be the motive.
Some of the people who benefited from the inheritance knew the farmhouse and the family well.
Some of these people were very familiar with guns and regularly shooting animals etc.
They may have done it personally or employed a person or persons to do it for them.