Perhaps you were unaware that this particular area of Essex is renown for witchcraft? However, as Jeremy took so long to bring his plan to 'perfection' he probably had no need of its assistance. He could, for instance, have employed some Pavlov-type persuasion on the dogs -I wonder from whom came the information that they barked? I wonder who, of all who gave information re the daily routine of WHF, OTHER than Jeremy, was actually privy to the daily routine. Of course there were risks. NOTHING is risk free. Jeremy must have reckoned that he could handle any he could think of. Those he couldn't, he naturally wouldn't have worried about until it was too late. Rather like having to shoot Sheila twice, actually. It's possible, even in the real world to (almost) get away with murder.
No Jane, it's not risk it's imperative.
It's imperative that Jeremy gets past the dogs without them barking and alerting Nevill.
It's imperative that while Jeremy slaughters Nevill and June first, they die quietly, so that the twins don't wake up and run amok about the farmhouse looking for places to hide and ruin his pre-laid plan.
It's imperative that his completely unpredictable schizophrenic sister is going to be completely compliant and allow him to shoot her in the neck, making it look like suicide.
None of these can be achieved logically, it can only be done magically, which is idiotic, that's why JB didn't do the crime.
If any of these obligatory scenarios don't work it's a total F**K-up. JB has got to know these things in his strategy and still find go through with it anyway. If you are suggesting he had contingency plans for the above, what were they? You're the originator of the "pre-laid plan" but failed to define what it was.
Hoots