They also happened to be the most effective individual tracking system ever devised.

It was the people-tracking aspect that had brought White to Court Number Two of the Old Bailey. The prosecution expert, he was there to demonstrate to the jury what that data proved.

The morning had been taken up with White’s presentation of his findings. Peter Epstein QC had questioned him in a way designed to guide him in his explanations to the jury. Cell-site data can be complicated and so it was essential that it was explained clearly. But Michael could see that Epstein’s guiding hand was unnecessary; White could do this in his sleep.

He had begun by explaining how the mobile-network operated, and how its data could be used to locate the position of a mobile phone at the time of that phone’s use. To do this he handed out a detailed map of a random location in West London with a single mobile-phone mast in the centre. The jury then watched as White used a marker pen to draw a line upwards from the cell mast to the edge of the map, another diagonally down to the left, and a third diagonally to the right, dividing the map into three sections, with the mast at the centre.

These sections, White explained, were the mast’s cell zones: A, B and C.

Michael listened intently, making sure all the information was presented clearly. White went on to explain that any mobile phone used at a location inside the map’s right-hand section – cell zone A – would usually register in that same zone, because the telephone was designed to seek out the nearest available transmitter. But sometimes cell zone A would already be busy with as many users as it could take at one time, and so the telephone would seek out the next nearest transmitter. This might be in the lower or the left-hand triangles – cell zones B or C – of the same mast, or it could be a different mast altogether.

‘But the vast majority of the time,’ White had explained, ‘the cell zone that registers the call will be the cell zone in which the phone is physically located. So we can say that during the call, the phone itself was – most likely – within the geographic area covered by cell zone A.’

Michael had watched the jury throughout the explanation. All of them seemed to have followed so far. But Michael knew that things were about to get a bit more complicated.

With the general explanation done, White produced another map – less detailed this time and covering a much larger area of five miles – and once copies had been handed to the jury he began to mark up all of the cell masts within that five-mile area. Not for the first time, Michael was amused by the jury’s surprise as mast after mast was identified; few realise quite how many of the things are hiding in plain sight.