Shaw opened a door and Shepherd followed him out. The unit was based in a container, the same size and shape as the ones used to carry goods on ships. There were two in a large hangar. Both were a dull yellow, with rubber wheels at either end so that they could be moved around, and large air-conditioning units attached to keep the occupants cool. The hangar was at RAF Waddington, four miles south of the city of Lincoln.

Shaw headed for the hangar entrance as he lit a cigarette. On the wall by the door was the badge of 13th Squadron – a lynx’s head in front of a dagger – and a motto: ADJUVAMUS TUENDO, ‘We Assist by Watching’. It was something of a misnomer as the squadron did much more than watch. Shaw blew smoke at the mid-morning sky. ‘It was like he had a sixth sense, wasn’t it? The way that sniper moved.’

‘Could he have heard the drone?’

Shaw flashed him an admonishing look. The men of 13th Squadron didn’t refer to the Predators as drones. They were RPAs, remotely piloted aircraft. Shepherd supposed it was because without the word ‘pilot’ in there somewhere, they might be considered surplus to requirements. Shepherd grinned and corrected himself. ‘RPA. Could he have heard the RPA?’

‘Not at the height we were at,’ said Shaw.

‘Must have spotters then, I guess.’

‘The two men with him were eyes on the target. They weren’t checking the sky.’

‘I meant other spotters. Somewhere else. In communication with him via radio or phone.’

‘I didn’t see any of them using phones or radios,’ said Shaw.

‘True,’ said Shepherd. ‘But he could have a phone set to vibrate. The phone vibrates, he grabs his gun and runs.’

‘Without warning his pals?’

‘He could have shouted as he ran. They froze. Bang.’

‘Our target was one of the guys with him. The Brit. Why are you so concerned about the one that got away?’

‘Usually snipers have just one spotter,’ said Shepherd. ‘Their job is to protect the sniper and help him by calling the wind and noting the shots. That guy had two. Plus it looks like there were more protecting him from a distance. That suggests to me he’s a valuable Islamic State resource. One of their best snipers. If he got away, I’d like at least to have some intel on him.’

‘The Brit who was with him. How long have you been on his tail?’

‘Khan’s been on our watch list since he entered Syria a year ago. He’s been posting some very nasty stuff on Facebook and Twitter.’

‘It was impressive the way you spotted him coming out of the mosque. I couldn’t tell him apart from the other men there.’

‘I’m good at recognising people, close up and from a distance.’