Willoughby-Brown pressed the button on his transceiver to get to the frequency Aspden was using.

‘Bravo Two has eyeball. On the A40 heading east.’

‘Roger that Bravo Two,’ he heard Aspden say. ‘Bravo One, prepare to take over eyeball.’

‘Bravo One preparing to take eyeball.’

Bravo 1 and Bravo 2 were the motorcycles being used in the surveillance. Bravo 1 was a small moped. A Plexiglas screen was attached to its handlebars with a taxi route clipped to it. Anyone seeing the bike would assume it was a wannabe black-cab driver learning the Knowledge. Bravo 2 was a high-powered Kawasaki bike, its rider outfitted as a courier.

There were two nondescript saloon cars, one of which was now a quarter of a mile ahead of the Prius, the other about the same distance behind. The bikes would take it in turns to keep the target car in sight while the two cars would be ready to take up any slack if necessary.

Willoughby-Brown pressed the transmit button. ‘Mike One is on line,’ he said.

‘Welcome aboard,’ said Aspden.

‘How are we looking?’ asked Willoughby-Brown.

‘So far so good,’ said Aspden. ‘From the way the Prius is being driven it doesn’t look as if they’re taking any counter-surveillance measures. They’re braking in plenty of time to stop at red lights, they indicate at every turn, and they’re at a constant speed most of the time.’

Willoughby-Brown pressed the button to talk to the driver again. ‘Tim, close to the A40 would be good.’

‘No problem, sir.’

Willoughby-Brown called Shepherd’s number but it went straight through to voicemail. He didn’t bother leaving a message. Shepherd would know what it was about.

Malik clicked the indicator to show he was turning right. Off to his left were the high brick walls of Wormwood Scrubs. He drove onto the A3220 and prepared to indicate another right turn, this one taking them into the huge Westfield Shopping Centre. He glanced into his rear-view mirror but didn’t notice the motorbike that was tucked in three cars behind.

He joined the queue of cars waiting to drive into the massive shopping centre: the size of thirty football pitches, it was home to more than 250 stores and parking for 4,500 vehicles.

‘Bravo One, Tango One is turning into the Westfield Shopping Centre,’ said the voice in Willoughby-Brown’s ear.

‘Stay with him, Bravo One,’ said Aspden.

‘Charlie One, Charlie Two, head for Westfield. It’s going to be crowded in there.’

Willoughby-Brown frowned. Attacking Westfield Shopping Centre with a rifle didn’t make much sense. A rifle was a surgical weapon, designed for taking out single targets. It was hardly the weapon of choice to cause carnage in a crowded shopping centre. And if that was their plan, the jihadists would find they had bitten off more than they could chew. Since the Islamic State terrorist attacks in Paris, the British government had increased security at shopping centres across the country. Armed police in plain clothes mingled with shoppers around the clock, and at times of high terrorist alert they were supplemented with armed SAS troopers. They would make short work of one man with a rifle. It would be different if they had been using explosives but there was no evidence of that.