Where was Jeremy when Neville supposedly phoned him and the police? I assume there is no corroboration for this?
Jeremy WAS at home at the time he (Jeremy) called the police. The number was traced to his home.
The call from Nevill to Jeremy (the established alibi) could not be verified by the authorities. Any call from Nevill to the police is merely speculative.
The general assumption (assuming JB's guilt) is that made the call from WHF to his own home, or left the phone off the hook to look like the call was made. Then rapidly got home to make a call to the police saying "my father's just called me"
The distance to home across fields is 1.7 miles, or 3.1 miles by road/track. It is alleged he used a bicycle to make the journey across fields, but zero proof of this exists (to my knowledge) and no tracks were every found.
IF he carried out the killings, he need to get home, make the call to the police, then get himself cleaned up in order to meet the police at WHF (which MIGHT account for him seeming to 'take his time' arriving).
That said, police witnessed him driving 'slowly' (about 29mph) - which could be argued as counter to the clean up theory, why didn't he rush, having lost time cleaning up? or, was it a cunning ruse to explain the slow arrival time?
When he made the call to the police, it was a gamble - the police COULD have said "okay Mr Bamber, there's a car close by, we'll come and pick you up and take you to WHF with us". Did he consider that scenario? was it so well planned? was it more spontaneous than we know? lots of questions.
I am sure he could run 1.7 miles in 15 minutes, but across fields in the dark might be tricky.
Nobody commented on anything unusual about his appearence when he met the police, other than to suggest he looked well dressed (which was later questioned and described as - trainers, jeans, shirt and two jumpers, so not exactly Saville Row!).
Considering come comments along the lines of "Nevill must have put up a hell of a fight in the kitchen" it seems both Jeremy AND Sheila showed little evidence of a fight, with Sheila looking the more likely than Jeremy.