I was wondering why Nevill Bamber would ring his son and came up with the possibility that he didn't. Maybe it was someone else who made that call, someone who wanted Jeremy Bamber to go to White House Farm?
This has been suggested previously along with the possibility NB might have been forced to make the call.
I think NB made the call of his own volition. Reasons:
- no evidence shots had been fired at the time
- as you've pointed out firearms were not stored in accordance with the law
- re above NB was a magistrate
- according to independent witnesses eg CC and the relatives, the Bambers did not discuss SC's illness with anyone
- police cars did not have navigation systems and the road leading up to White House Farm is obscure. NB may have thought to get JB in loop to make sure they got there.
- According to a relative NB referred to the local police as Dad's Army so it seems he did not have much confidence in them.
- opium poppies were growing on the farm which appear to be somewhat of a mystery in terms of what they were being grown for and for whom
- a small piece of cannabis resin was found in a safe within the farmhouse