But the thought of his 'nutty' sister brandishing a gun and potentially firing it, didn't!?
I am talking from his perspective of stage 2 of his plan.
When at bourtree he could have phoned 999 instead of looking up the number for Chelmsford Police.
He is not incriminating himself by phoning 999, if anything he is bolstering his stance of the " concerned son " by ringing 999. He was scathed upon for looking up a police station in the phonebook.
So you have to ask yourself why was he from his perspective hesitant to phone 999. Or why he didn't phone 999. There must be a reason why he didn't.
The farm would be swaming with police eventually anyway.