Next out was the power cable, connected to a lithium battery inside his day sack. Charlie plugged it into the pyramid. There was a moment's delay as jaws inside the pyramid clamped round the bit, and then it began to turn, so slowly it almost seemed not to be moving. The only sound was a barely audible, low-frequency hum.

There was nothing we could do now but wait as it started to work its way quietly, slowly, methodically through an inch of wood, a sheet of tarpaper, and about half a centimetre of plasterboard. I moved against the wall to make myself as small a target as possible if anyone looked out of a window. My right hand lifted my fleece and rested on the grip of the pistol pancake-holstered on my jean belt. My left pulled the zipped-up front over my nose for warmth.

This kit worked on the same technology they used in neurosurgery; if you're drilling through a skull it helps to be doing so with something that stops when it senses it's about to hit the cranial membrane. Our one behaved the same way when it was just about to break through the final layer of paint or paper. And – so it left no sign – it automatically collected the debris and dust as it went.

Charlie disconnected the power and pulled out the bit, then took out a fibre-optic rod with a light on the end. He moved it down through the pyramid, just to make sure he wasn't about to break through the stud wall. Everything seemed to be fine. He removed the fibre optic, reinserted the drill, and reconnected the power. The gentle hum resumed.

It moved quicker as it hit the tarpaper, then slowed again as it encountered the plasterboard. Charlie stopped it again and repeated the operation with the fibre optic.

I looked over at Half Arse. He was lying on his back with his feet nearest the wall, his pistol resting on his chest, pointing up at the first-floor windows. He must have been freezing his arse off – or what was left of it. I thought about the Americans in the Pods, drinking coffee and smoking cigars while they watched our progress. Most of them were probably wondering why the fuck we didn't get a move on.