Good observations. 
That´s why I never do!
Two erroneous clocks in the mix is a turnoff for me. 
P.S. criminals who plan their crimes ahead, as Jeremy is alleged to have done, often have the timings rehearsed down to minutes, like, 9:42 am (just an example).
That raises red flags - not that someone innocent does not remember exact timings. And as Jan says, the times are all over the place for everyone concerned in this case - everyone, not just Jeremy.
Jeremy did have the times sorted. Neville phoned and then he rang a police station over 20 miles away. Choosing the ninth best option. That is not a crime.
Several pages of his police interviews have him unsure of whether he called Julie before or after the police. Eventually opting for 'after ringing the police'. Again not a crime.
The problems start decades later with him claiming Neville rang the police. Meaning his timings have been wildly changed to assist his claim.
Neville's 8 word, 5 second call to Jeremy also becomes more weird as Neville was able to spend several minutes ringing the police 16 minutes later. Why ring Jeremy at all for just 5 seconds ?
Neville's call to the police also means Jeremy phoned Julie before the police.