In a guilty scenario, he would call the police once he was prepared for to. Also in such event the call from his father never happened, thus no real time gap between the calls. So what is there to delay?
This idea of delaying the police contradicts the claim that Jeremy drove slow because he wanted the police to arrive at the scene before he did.
If he's guilty, he'd want a delay because Sheila needed time to shoot everyone after Nevill phoned him. Also, he wouldn't want the police getting into the house too soon because someone might realise that the five of them had been dead for longer than was possible if Nevill had phoned Jeremy.
I've always found it odd that the time of death wasn't slightly more specific.