I can only think he was trying to buy time. What he'd done, surely couldn't have not shaken him? If only by way of making him question that he'd got it right, wonder if he'd gone over the top with his father, be concerned about the two shots to Sheila. Wonder if he'd covered himself enough to get away with it. Remember, once that call went through to police, the fat was in the frying pan, so to speak. There was no going back.
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.