Hoskins was not enthusiastic. ‘There’s bound to be thousands. That’s a heck of a busy road.’

‘I know but we can narrow down the time slot. And I have a couple of ideas for which hits will be relevant.’

Gillard and Hoskins sat side by side at twin computer terminals, scrolling through pages and pages of registration numbers, hits that had been made on the two number plate recognition cameras on Hampton Court Road. The detective chief inspector sorted it by the timestamp on the left of each entry. ‘I want you to check every hit from half eleven at night until three the next morning.’ He scrolled up screen after screen. ‘Okay, that’s twelve pages of records. We’ve got one camera a mile to the left of the Tagg’s Island turn-off, and one on the Hampton Court bridge 400 yards to the right.’

Hoskins sagged at the prospect as he hit the print button. ‘That’s over 600 records, sir. What are we looking for?’

‘Ignore any vehicles that hit both cameras in quick succession, that’s through traffic. Prioritise any that tripped only one camera. The best would be two hits on the same camera pair, once in each direction. That might indicate a there-and-back journey to the island, or somewhere close to it. We may pick up some residents too, but that’s no bad thing. Anyone about at that time might have seen something.’

Hoskins nodded. He never failed to be impressed by his boss’s analytical mind. There was a tool on the system that would flag up vehicles that had hit two specified cameras. Gillard showed him how to use it, and clicked search.

‘That’s not so bad,’ Gillard said, as a result came up. ‘You’ve eliminated 579. That’s only twenty-seven to check. Bear in mind that some of these will only have hit a single camera once, but it could still be through traffic.’

‘Yeah, the A308 continues north from the roundabout, and those vehicles would miss the camera on the bridge.’

They both squinted at a Google map that showed the three main arterial routes joining at a roundabout. ‘Anyway, see how you get on with that,’ Gillard said.

Six o’clock on Saturday afternoon. The lurid photographic enlargements from CSI were laid across Gillard’s desk, like a pack of horror cards, eight inches by ten of glossy gore. A life ended, presumably, in terror and agony. Someone nobody missed, and somebody hated. Disposed of like rubbish, tossed into a river. What had caused that diamond pattern on his body? In any criminal investigation there are theories and speculation, ideas and hunches. None of it gets you anywhere until you have some firm facts, some clear evidence, to act as foundations. All he had so far was a splash.

The ringing phone rescued him. Dr David Delahaye.