And so they had spent the past two days meandering up the Rhine, sometimes ahead of the Wilhelmina Rosen, sometimes far in her wake, one or other of them in the cockpit with a pair of powerful binoculars, watching the movements of the three men on board. Every couple of hours or so, Karpf and Marijke would exchange phone calls, keeping each other up to date with the movements of the barge. The first night, it had motored until midnight, then anchored offshore, out of the shipping channel. Marijke and Tony had had to carry on downriver for another mile or so before they found a wharf where they could tie up. Marijke had insisted on sleeping for no more than four hours, lest they miss their target. ‘I’m beginning to think the German police had a point about the difficulties of maintaining surveillance on a boat,’ she’d said wryly as she zipped herself into her sleeping bag.

‘At least we know he’s not murdering anyone tonight,’ Tony said. ‘He can’t get the car ashore from there.’

Marijke had been huddled in the cockpit over a steaming cup of tea when the Wilhelmina Rosen had passed them just after six. She called to Tony to take the helm while she cast off, and they were soon back on the trail. The day’s journey brought them to the Dutch border, and the barge made its way into the first commercial harbour on Dutch territory, Vluchthaven Lobith-Tolkamer. ‘What do we do now?’ Tony asked.

‘It’s an hour since I put my team on stand-by. They should be able to get here very quickly. Now, according to the chart, we can use this harbour too,’ Marijke said, turning the helm. ‘We watch where the Wilhelmina Rosen moors up, I put you ashore, then I go and find a yacht mooring, no?’

It was easier said than done. They managed to keep their objective in sight, but there was no easy way for Tony to go ashore nearby. The only possibility would have involved climbing a dozen feet up an iron ladder set into the harbour wall, and Tony had to acknowledge that was far beyond his present capabilities. Eventually, Marijke found a pontoon where he could scramble on to dry land, but by then they were both in a ferment of frustration and anxiety.

Tony hurried back to where they’d last seen the Wilhelmina Rosen, a task that was easier in theory than in practice because of the pontoons and moles that stuck out at apparently random angles to the main wharves. Eventually, he found himself at one end of a long jetty. Towards the end of it, he could see the Wilhelmina Rosen. With a sense of relief, he saw that the Golf still sat on the stern roof.