Jeremy was asked "Approximately how long would it have taken you to find the number and get through?" He replied "Ten minutes at the outside." That's not the same thing as saying it took him around ten minutes.
The idea that he should have immediately dialled 999 and that his failure to do so indicates guilt is not really supported by anything, other than the belief of those who believe JB guilty that it is what they would have done.
Is this true though? Why would Jeremy assume the situation was so urgent that he should dial 999? If Neville had been able to call Jeremy then why would Jeremy assume that he was required to dial 999. Surely it would be perfectly reasonable for JB to believe that Nevill either didn't want the police involved or had called himself if he had. If Nevill was able to use the phone to call Jeremy then he was also able to call the police himself so why would JB imagine that he himself should immediately dial 999.
It is only the fact that the phone went dead and that it was engaged when attempting to call back that made calling the police an option according to JB's version.
Whether or not this version is believed obviously depends on your stance of guilt or innocence but to disbelieve it on the basis of JB not dialling 999 is not the best piece of reasoning.
That Nevill kept family affairs private is known, well documented and presumably accepted by all who post on here, so why is it considered not believable that JB would not call 999?