‘I’m not sure we’ll get anything,’ Townsend said. Using his cursor he outlined an enhancement focus box around the mirror, clicked on maximum magnification, and blew it up until it filled most of the screen. At this magnification it looked like a jumble of grey circles. ‘The CCTV is from a pretty good domestic system, but there’s not much more than a hundred pixels for the mirror. You won’t see anything.’ He clicked to let the sequence run, which simply illuminated the occasional lateral passage of brighter then darker blobs.

‘It’s not entirely useless,’ Gillard said. ‘We can distinguish between left-to-right and right-to-left movement, and each pairing of light and then darker blobs indicates a vehicle passing. So we can still count the passage of traffic past the Shere Road bus stop, which is just up thirty yards or so to the left.’

‘I’m not sure how much that is going to help,’ Townsend said, looking up at his boss.

‘It might do. Let’s go back to basics. I’m assuming we believe Singer’s account of Beatrice leaving his home, because we have an independent witness who saw her at the bus stop just before eleven p.m. This elderly man, Edward Lightfoot, says he didn’t stop, but presumably somebody else did shortly afterwards. I don’t believe Beatrice would have waited more than an hour when it was clear the bus wasn’t going to come and she was unable to get a taxi. So she might have hitched a lift. So between eleven and midnight, we can count the vehicle movements in each direction. That in itself is useful. But given that the bus stop is only thirty yards out of shot, we should also be able to detect whether vehicles were slowing down to stop, by measuring the passage of the dark and light blobs across the screen. Does that make sense?’

Townsend was nodding in agreement, wide-eyed with appreciation for cold, hard logic.

‘I’ll send in Hoskins to assist. Take an hour and see what you can manage,’ Gillard said, patting the young detective on the shoulder. ‘And do try to get some more sleep, young man. I need you fully awake and on the ball.’

It was only half an hour later when Rob Townsend messaged Gillard to say he’d managed to make some progress. When the detective chief inspector arrived at the surveillance suite, Townsend proudly displayed a much clearer picture of the blind spot mirror. Though vehicles still moved across as blobs of light and dark, it took much less imagination to see them for what they were.

‘How did you do that?’ Gillard asked.

‘The enhancement focus box has a probability sampler, which allows the subdivision of individual pixels. Though the camera never recorded this kind of detail, the processor can do a statistical likelihood of what the picture was trying to show. It’s a bit like predictive texting, where the computer suggests what to say.’

‘That’s great.’

‘I also think I know when Beatrice was picked up at the bus stop.’ He pointed at the screen, as he ran a five-second segment of the footage. ‘You can see this dark blob which slows down, and there’s even a light grey blob behind which could be brake lights. The time is 11.17 p.m. There is no other vehicle on her side of the road which shows the same characteristics.’