I have never written to Jeremy, and I doubt I ever will. I am in doubt whether he committed the murders and wouldn´t know WHAT to say to him because of that!

I think that Jeremy could have come to the conclusion that Nevill might be able to talk the local coppers out of contacting higher authorities regarding Sheila´s "episode" (read Child Protective Services). If he called 999 some strangers would come to the farm.
Maybe.
BTW, the sentence which Jeremy allegedly said to Julie "Everything is going well, something is wrong at the farm", simply does not make sense. It does make sense if you add a "but".
"Everything is going well (with me), but something is wrong at the farm. (What should I do?)"