There was a third option – high altitude, no opening – but the SAS tried to avoid that manoeuvre as far as possible. A twelve-stone trooper with fifty pounds of kit travelling at a hundred and twenty miles an hour made a hell of a mess on impact. Shepherd flashed back to his first HALO jump high over Salisbury Plain in the West Country. He had been one of six on the course, under the wing of a grizzled sergeant who’d been with the Regiment for going on fifteen years. As the sergeant was lining up Shepherd and the others, he’d stuffed a large piece of parachute silk into Shepherd’s pack. When it was Shepherd’s turn to exit the plane, the sergeant had tapped his shoulder. Shepherd had turned, seen what looked like a ripped chute, and the sergeant had pushed him out. It was the longest two minutes of Shepherd’s life, hurtling towards the ground at terminal velocity in full kit, not knowing if he had a faulty chute on his back. It had been a week before he’d seen the funny side.

The red light on the bulkhead flicked off and the amber one went on. The troopers took off the oxygen masks linked to the plane’s supply and replaced them with their own, then shuffled towards the front of the plane. The loader nodded at Gannon and opened the crew exit door. Shepherd stood next to Gannon, breathing slowly but deeply, the adrenaline coursing through his veins, heart pounding, stomach churning as his body geared up for what was to come. Part of him was terrified at the thought of jumping out of a plane six miles above the ocean, but another part relished the fear. He was doing what he’d been trained to do.

The engine noise died down as the pilot pulled back on the throttles of the remaining two engines to idle and the nose of the plane went up. Shepherd almost lost his balance and Gannon reached over to steady him. The Nimrod shuddered as the pilot fought to keep it steady at the close-to-stalling airspeed. Shepherd looked through the open door at the black night sky, peppered with a million stars. Far below was a layer of thick cloud, from fifteen thousandfeet to six thousand, which meant gliding through nine thousand feet of close-to-freezing water vapour. That and the landing would be the most dangerous phases of the operation. It was easy to get disoriented in cloud and there was a risk of collision – dealing with tangled chutes in cloud at night was an interesting proposition that Shepherd was keen to avoid.

The amber light winked off and the green light went on. Gannon grinned at Shepherd, behind his mask, then jumped out of the exit door, thrusting out his arms and legs in a starfish pose. Shepherd took a deep breath and followed him. He gasped as the wind tore at him, pulling, twisting, pummelling. He fought to keep his arms and legs out as he fell through the slipstream. As soon as his descent stabilised he pulled in his arms and legs slightly, adopting the stable position, his back arched so that his centre of gravity shifted towards his stomach.