‘I’m not sure,’ Tilling said. ‘It was after midnight. I got to the bridge, and it was noisy. Rap music or something like that, coming from a car parked in the middle of the bridge to Tagg’s Island. I waited on the north bank, unpacked my kit and waited for them to go. I couldn’t decide whether to drive around to Hurst Meadows park on the southern bank. The park is bigger, so it’s easier to get away from people, but the parking would be shut there, and it’s further to lug the kit.’

‘What time did you leave the north bank?’

‘I gave up before two a.m.,’ he said. ‘They showed no signs of leaving.’ Wickens looked at his notes. The western camera had recorded the Golf passing eastbound at 00:47 a.m. and returning westbound at 2:07 a.m.

‘Can you describe the vehicle that was on the bridge?’

‘It was a white BMW.’

‘Did you see the occupants?’

‘There seemed to be several, because of the conversation. There were two black guys stood looking over the water, but they were also talking to somebody inside the car.’

Wickens felt he was really getting somewhere now. ‘Can I ask why you didn’t call us with this information? It could be quite important.’

‘It was just some people on the bridge,’ Tilling said, looking at his hands. ‘I should have listened to Mum. She said to ring in.’

Wickens could see that Tilling’s departure time didn’t quite tally with the camera record. There would be nearly ten minutes unaccounted for after Tilling said he’d left. Yet it would only take a minute to drive that distance from the Tagg’s Island bridge to the western camera. He decided that it was probably nothing. The white BMW was a more interesting prospect.

‘Do you want to see the otters?’ Tilling said.

‘Why not,’ Wickens replied. At least it might explain Tilling’s motive for being there.

Tilling led him to the sun lounge at the back of the house. ‘This is my office,’ he said unnecessarily. The room was packed, but markedly less neglected than the rest of the house. Computers, laptops, screens, modems, keyboards on and under tables, resting in heaps on chairs and boxes. The whole floor was criss-crossed with networks of cables, over which plywood duckboards had been laid. The electrical sockets were crowded with plug adapters. The whole place had fire hazard written all over it. Tilling picked up a large camera with a telephoto lens attached and, after fiddling with the screen at the back, showed it to Wickens. A five-second video clearly showed an otter sitting on a riverside log. It preened itself, before staring in the direction of the lens and then sliding sinuously into the water.

‘You took that?’ Wickens asked.

Tilling nodded. ‘Last year, in the winter. You have to be very patient. They’ve got very keen senses.’