With all masts highlighted, White then circled one mast in particular. He explained that this was the same mast they had looked at on the previous map and then drew the same three lines emanating from it: a line upwards, a line down to the left, and a line down to the right. As the map was of a far larger area, however, none of these lines extended beyond an inch in length. Done, he went on to highlight a main road that ran through the mast’s cell zone A. He then traced the route of that road through the full five miles covered by the map, which, by the end, was very far away from the initial identified cell mast.

‘Now imagine that the call was made from this street,’ he had continued, indicating the highlighted road, ‘and that the caller was in a car travelling south. If that call continues during the journey, at some point it will no longer physically be within cell zone A – the right hand of our first mast, which we’ll call Mast One. Once Mast One is no longer close enough, the telephone will look for the new nearest mast. And when it finds it, it will jump to that mast. So, if the caller is driving along the road highlighted and reaches this point . . .’

White indicated a spot that was a mile or so further along the road.

‘. . . then in all likelihood the signal will be picked up by this mast . . .’

He indicated a different mast, the one now closest to his hypothetical car, and he identified the bottom cell zone section it would now be travelling through by deftly adding the three one-inch zone lines as with Mast One.

‘. . . which, using our same 120-degree triangulation, is cell zone B of what we will call Mast Two.’

Michael watched the jury nod in understanding. It was not just courtesy.

Still with him so far, he thought. Let’s hope they can keep up.

White continued, demonstrating the likely cell masts and cell zones for every half mile of movement along the highlighted road. When he was done the jury had a list of mast hits on a southern route which, White explained, would be strong evidence of that imaginary journey.

‘But what if one or some of those masts or cell zones were working at capacity, Mr White? What if one or some of them had taken all of the simultaneous users they are able to carry?’

Epstein knew the potential lines of attack. His questions were designed to close them.

‘In those circumstances, where would the phone uses be registered?’

‘In those circumstances, Mr Epstein, the telephone would look for the next nearest transmitter. Which could be on the same mast but in one of the other two cell zones. Or it could be a different mast entirely. Still relatively nearby, but not quite as close.’

‘And how likely is it, for example, that the nearest available mast would be, say, over a mile away from where the phone is being used?’