I've realised there is a flaw in my scenario in that if Jeremy uses an answerphone as suggested, that would then pinpoint the call at a particular time that would, in turn, make it impossible for Jeremy to then call the police from Bourtree Cottage within, say, 10 minutes of the alert from Nevill.
I think this is a point that has to be researched further because I don't remember how 1980s phones worked. Is there anybody else here who knows the answer and can provide reliable sources?
Here we are assuming that Jeremy decides he has to stage a call (rather than just make one up), and notwithstanding that billing was non-itemised at that time, he assumes that there may be a way of pinpointing when calls are made, so he has to take precautions.
The obvious thing for Jeremy to do in this situation would be to:
(i). ring Bourtree Cottage from the farmhouse;
(ii). ensure there is a ring tone, then leave it to ring;
(iii). then terminate the call roughly 50 minutes or so later at Bourtree Cottage;
(iv). then ring the farmhouse from Bourtree Cottage, ensuring there is an engaged tone at the other end;
(v). then terminate that call.
Was this technically possible at that time?