As they approached the castle, Ben was suddenly able to make out the faint glow of its lights through the swirling whiteness. But as the walls drew closer, instead of heading straight towards them he veered off to the right and began cutting a wide circle around. Towers and ramparts loomed above them in the murk. Ben pressed on harder, steering parallel with the east wall and heading straight towards the mountainside that rose up behind it to the north. He could only hope that the howling wind would make the roar of the Polaris’s engine inaudible from within the castle buildings.

Now the buggy was reaching the steep terrain of the mountainside, tyres biting and slithering over rocks and ruts. Ben and Boonzie were pressed back into their seats by the sharp upwards angle of their ascent. He kept his foot down on the gas and fought to keep the leaping, bucking vehicle’s nose pointing straight ahead as they climbed up and up. It was a long way to the bottom, if he lost momentum or allowed the vehicle to tip over. He gritted his teeth and kept on going. Soon the castle seemed to shrink below them, dwarfed by the vastness of the mountain. Ben could see the lights of its windows and the outline of the main courtyard from above.

Which meant that they had climbed far enough. The next phase of the plan was the most risky. At Boonzie’s signal, Ben brought the buggy around in a curve so that they were pointing parallel with the rearward-facing north wall of the castle. The angle of the slope was hair-raising. One more degree of sideways cant, and the buggy would have tipped over and not stopped rolling until it pulverised itself and both men inside it on the rocks below. Another signal from Boonzie, and Ben rolled to a halt. Neither man spoke, or needed to. Boonzie jumped out, grabbed the cable reel from the back of the buggy and rolled out a few feet of detonator cord. Working fast, he wedged the loose end of the cord under a heavy boulder so that it was pinned in place. Then he jumped back aboard the buggy clutching the cable reel, letting it unravel as Ben continued on their precarious path. As intensely cold as it was, they were both sweating. But the buggy clung like a spider to the rock face, and after a hundred anxious metres all of the cord had been used up.

Boonzie pinned that end under a second boulder. With the detonator cord stretched sideways across the sloping face of the mountainside above the gables and towers of Stuart’s castle, it was time to begin the next phase of the plan.