‘They’re ready for the off.’

Captain Burrows had calculated with naval precision that it would take seven minutes and thirty-five seconds to taxi out to sea, gather enough power for the lumbering Supermarine Scapa to take off, reach the agreed height and then fly to the starting position to the south, over Harwich. They would pass Bawdsey heading north over the sea precisely three miles from the coast and would then circle and return, repeating the pattern once more so that the system could be tested twice in both directions at three, ten and then twenty miles out.

‘Transmitter power on?’

A voice squawked down the phone. ‘Roger that.’

Vic visualised the arcs of radio waves fanning out around the coast, floodlighting the air with invisible radio pulse energy, like the three-dimensional spider’s web Watson-Watt had described at his very first briefing. Was it a friendly bee or an enemy wasp about to fly into the lair? In the next few minutes, if their calculations were right, they would, for the very first time, be able to tell. If their system worked, it would provide the perfect defence against an enemy: a magic eye, secret, invisible and brilliantly effective.

Slowly, a line emerged, a white squiggle worming its way horizontally across the black screen, left to right. He twiddled the left-hand knob, which identified range, looking for the spike that would indicate the presence of an aircraft or other object reflecting the beams being transmitted from another building half a mile away. Three miles, ten, twenty, thirty . . . nothing. A good sign. They didn’t want other planes in the air confusing their picture, not today.

Forcing himself to work slowly and patiently, he turned the dial the other way, scanning the empty air. The first time he’d been trained to use this screen, back in the first months of his attachment, Vic had found it utterly confusing. How could anyone distinguish anything sensible from the apparently random streaks and quivering smears that appeared and disappeared in an instant? But he’d learned, in time, to understand the subtleties and to interpret them with accuracy. It was like reading sheet music, or Greek, or a child learning language for the very first time.

‘They’re up,’ Scott said, and then, a few moments later. ‘At height . . . turning . . . approaching now.’

Vic’s heart began to thud in his chest. He twiddled the range dial. No sign. Seconds passed, and then, at six miles, he located a deep V cutting through the wriggly line.

‘I’ve got her,’ he shouted.

He turned the right-hand dial, the one that would indicate bearing, the direction in which the plane was travelling. Yes, it was heading north, parallel to the coast. He twiddled both knobs now, trying to detect the modified signal.

Then he saw it, and his heart did a double beat. The plane, flying parallel to the coast, was approaching and now, at five miles range, the sharp point of the V shape bisected, forming a double V. This was the signal of interference they’d been praying to see, that they had spent so many months working on, created by the deceptively simple design of dipole wires installed onto the aircraft.