I often wonder if he toyed with the idea of waiting till the morning.
If the workers couldn't get in touch will Nevill they will not attempt to go in the farmhouse and assume Nevill is preoccupied. However they would be alarmed by the fact that Nevills cars and vans would still be on the premises.
Either way the workers will wait till Jeremy arrives.
They have then the option of kicking the door down or phoning the police.
The trouble with this situation is Jeremy can't heavily insinuate the guilt as Shelia, it would be more a " somethings not right here " scenario then " she's gone crazy with a gun, even if that's what they would find.
He needed an Alibi, this way he has No Alibi other than arriving in the morning and finding the event.
If he receives a phone call from WHF from his Father and shows there was a call from WHF to GH, this then backs up his claim that he was at home. All he has to do then, is manipulate the times around to suit. By not calling 999 leaves him breathing space and a delay tactic. He can then further cement his Alibi by arriving after the Police, and by this time, it then enables him to build on the story and picture while he’s outside standing with the police, he sort off becomes a victim of what’s happening.
Both ways have risk, and one can never be sure of the right way, but if someone rings you at home, and asks for your help, means that person is alive and someone else is carrying out the murders?