‘Not exactly everywhere because there are still places in the United Kingdom that don’t have CCTV coverage,’ she said. ‘But certainly we can see what major roads they have been on.’

Shepherd sipped his coffee. ‘You know where this is going, don’t you?’

Button shook her head. ‘Tell me.’

‘They’ll be tying this data to facial recognition systems and then you’ll be able to follow every single person, no matter where they are.’

Button nodded slowly. ‘You’re probably right,’ she said.

Shepherd opened his mouth to speak but he was interrupted by a female technician. ‘I’ve got a hit!’ she said excitedly. ‘On the M3 outside Southampton.’

‘On the screen, please, Daisy,’ said Button.

The technician’s fingers played across her keyboard and a view from a motorway CCTV camera filled the screen. Among the traffic was the truck and its container. ‘Freeze and zoom, please.’

The image zoomed in on the truck and they could clearly see the two black men in the cab.

At the bottom of the screen was a time code. It was three hours after the Athena had docked.

‘Right, that’s a start,’ said Button. ‘Let’s concentrate on the M3. Where is that exactly?’

‘Five miles from the city, heading north,’ said the technician.

‘Right, let’s focus our search on the M3, and of course I don’t have to point out that the M3 heads straight to the M25, which circles London.’

It was a relatively simple circuit but the two men wanted to be absolutely certain that it would work so they checked and rechecked it before activating it. The bomb had been well constructed by experts – Russian technicians who had been well paid for their work. They had put the bomb together in a disused factory on the outskirts of the seaport city of Aden using radioactive waste that had been obtained from a group of Chechen rebels. The Russians had designed and built the detonating circuit but had kept the individual components separate to be assembled when the container was close to its target.

The radioactive material was sealed in aluminium barrels and they had used a Geiger counter to check that the radiation levels in the container were not dangerous, so long as they wore their government-approved radiation suits and helmets.

Both men had been born in Scotland and attended university there, but when they spoke they spoke in Urdu because they did not want the two Somalis to understand them. ‘It’s done,’ said the elder of the two. His name was Gadi Hussain and he had a degree in chemical engineering from Edinburgh’s Heriot-Watt University. That was where he had met the second man, Chishti Akram, who was studying electrical engineering. They had joined the university’s Muslim Society and after graduating had spent three months together in an al-Qaeda training camp in northern Pakistan.

Akram looked over his shoulder at the two Somalis who were sitting at a wooden table, eating leaves from a polythene bag.