Having examined the two tapes, they ran them side by side, and then started freeze-framing particular shots, shots of similar buildings taken from similar angles. On the top left corner of each was an hour-and-minute counter, and they used this to align the two tapes temporally, checking differences in light and in the quality of the shadows cast by the evening sun. Izzard never seemed satisfied, and would run a section again, sharpening the focus, enlarging a shot onscreen: this enlarging process was again of his own design and the unit he operated his own construction.

‘I haven’t perfected it yet,’ he admitted, though the results were, to Hepton’s eyes, impressive enough.

At four o’clock, Izzard suggested they pause for breakfast. Sanders was snoring, so they left him to his sleep and went outside. Birds were chirping hesitantly in the distance, and a few early cars and lorries were on the road. After the cool of the lab, the morning seemed already oppressively warm. Izzard walked with hands in pockets.

‘I think I can see it,’ he said.

‘What?’ Hepton asked, still coming out of his brief hallucinatory stage.

‘What it is you’ve been looking for. I can see it now.’ Izzard turned to him. ‘They’re not the same place, are they?’

Hepton smiled. ‘No,’ he said. ‘They’re not.’ He was pleased that Izzard could see it, too. If he could see it, then everybody could see it. It wasn’t just in Hepton’s mind. ‘I should have realised right back at the beginning,’ he explained. ‘One day I was watching Buchan and it got dark at a certain time. Then Zephyr was got at, and suddenly it was starting to get dark earlier at Buchan.’

‘Except that it wasn’t Buchan,’ Izzard noted.

‘That’s right,’ said Hepton. ‘That’s what this is all about. Someone doesn’t want us to see what’s really going on at Buchan. So instead they’ve rigged a lookalike.’

‘A mock-up.’

‘Yes. But a very good mock-up. A bit too good.’

‘How do you mean?’

‘I think it must be a real airbase, but not one of the other ones the USAF has been using. We’ve been watching all of those at one time or other. It must be RAF.’

‘Excuse my ignorance, but if all you do all day is watch these pictures, wouldn’t someone notice that it wasn’t Buchan?’

‘Not really,’ said Hepton. ‘For one thing, our remit wasn’t to watch the bases so much as watch their perimeters for protesters.’

‘Weird,’ said Izzard. ‘Not so long ago, they were protesting about the Americans being here. Now they’re protesting about them going.’

‘Besides,’ Hepton continued, ‘we mostly examine still photos, and still aerial photos look much the same. We would be checking for things that were different, not things that looked the same. We’re only technicians, remember. We’re not spies.’