Hi lookout (for later) I have followed this train of thought myself. If going with the argument that we can see or hear what we want to believe we are seeing or hearing, I can imagine a scenario where Jeremy woken from a deep sleep answers the phone and hears a voice that sounds like his father, saying things only his father would be likely to say by the time he puts the phone down he has an unshakeable belief it is his father.
I believe this could be a remote possibility, don't think anyone else does, apart from anything how did anyone shoot Sheila without a struggle? 
We have to accept Jeremy wakes up from a sleep in the first place,manages to get down the stairs before the telephone stops ringing,that Nevill at the other end knows the Goldhanger number by heart and chooses to telephone this number rather than dial 999 himself,that Nevill manages to get to the telephone in the first place in an unharmed state,that it's immature Jeremy who decides to telephone the Police and not keep things in the family as was Nevill's justification for telephoning Jeremy in the first place. Far more likely Jeremy uses his answerphone to place a call from White House to Bourtree Cottage,then retrieves the tape on his return.
As for Sheila,we are not dealing with a mother in the normal sense of the word that final week let's be frank. Sheila is in a moribund state as evidenced by the Saturday night party,her reaction to the visit of the electrician,her vacant stare in the Tiptree store. Her personality is that of gullibility,of being easily led,of being unaware of people around her or of differentiating between the concept of fantasy and reality. Whilst this in theory gave Jeremy Bamber the opportunity to commit the perfect crime and use Sheila as a scapegoat the Jeremy supporters must also come to realize that when awoken in the middle of the night and quickly hurried by Jeremy under some pretext into the master bedroom Sheila would not have the first inkling of what was going on until it was too late..
