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.

‘What is it with the leaves they chew?’ he asked.

‘That’s khat. It’s a stimulant.’

‘And it’s allowed? They are Muslims, right?’

Hussain chuckled. ‘They are Muslims but they are Africans. They don’t know any better.’

‘But they are shahids. They must go to meet God in a state of purity and cleanliness, surely.’

Hussain lowered his voice, even though he knew that the two Somalis could not understand Urdu. ‘They do not know, brother. They think that the bomb is going to be detonated by a mobile phone. They do not know that it is on a timer.’

‘And they will die without knowing?’

‘Inshallah,’ said Hussain.

‘Because they cannot be trusted?’

‘They are Africans, brother. Who can trust an African? You have to count your fingers every time you shake hands with them.’

He turned and flashed Crazy Boy a beaming smile. ‘We are ready,’ he said. ‘Everything is done. Help us close the doors.’

Charlotte Button paced up and down behind the technicians. On the screens on the wall behind her were six CCTV feeds showing the progress of the truck along the M3. One of the screens showed the truck leaving the M3 at the Basingstoke turn-off and since then there had been no sightings.

‘Come on, everyone, let’s find it,’ she said, even though she knew they were working flat out and that the lack of success wasn’t for want of trying. She rubbed her hands together and looked across at Shepherd. ‘At least Basingstoke probably isn’t the target.’

‘You don’t know that,’ said Shepherd. ‘You heard what Dr Wilson said, a dirty bomb anywhere in the country brings the economy grinding to a halt. And London is always going to be a harder target because . . .’

He was interrupted by one of the male technicians, who raised his hand. ‘I have a hit,’ he said. ‘Camberley, thirty minutes ago.’

‘Heading north?’

The technician nodded and put the recording on to one of the large monitors. The truck was in the inside lane, hemmed in by heavy traffic.