Thank you once again, Home Guard Guide 1940. I reckon that this is as late as reasonably possible. My detonator is the rickety combination of a dismembered disposable camera, crucially equipped with a flash, and a cheap mechanical egg timer that leans on the shutter button when it hits zero. It’s far less clever than it sounds, and at least as ugly. But I’ve seen the same set-up work.

There’s one last procedure. On the floor below the bench, lurking under a tarpaulin, is a large plastic ice-cream tub, loaded with a couple of kilos of nails and screws. I snap away the lid and paw at the fixings, feeling their dead weight in my hand.

‘Shrapnel (nails or bolts, for example) will maximize the casualties resulting from the blast.’

I begin to lower the device towards the mass of cold, sharpened metal. I could quite easily have done this in advance. But something prevented me, just as it seems to be holding me back now. I lay the cylinder on the bench, not in the tub but alongside it. I take a step backwards.

Standing a metre away, I feel a twinge of panic. A decision has to be made. I’ve pushed it to one side until now, but I can stall no longer. A sharp inward breath takes me by surprise. The smell of the shed – motor oil and creosote – ordinarily so comforting, is beginning to make me feel unwell.

For maximum effectiveness, the bomb wants to be in that tub. Placing it there condemns everybody within the blast radius. It would be like a hundred machine guns loosing off in every direction. But I’ve seen, and I’ve heard, just how savage this device will be. And that successful development model, as well as being smaller than this one, never had the shrapnel – even at such a distance, its presence would have put Sophia and me at serious risk.

Is it too much?

Make up your mind. Make up your mind right now.

There’s a split second when I can visualize it. Indiscriminate. Shards of steel tearing through people I don’t even know, peppering those unfortunate enough to just be close to the classroom at the wrong time. I feel the blood draining from my face. That’s not what this is about.

In one stride, I’m back at the bench. It is too much.

Two kilos of screws and nails shower to the floor with a crash. There’s no time to gather them up now, no opportunity to renege on the decision. The cylinder is loaded into the emptied tub, packed in place instead with hastily scrunched-up newspapers from the recycling pile.

Opening my rucksack wide on the floor, I lower the box inside, wrapping it all in the pillow I took from my bed earlier. Slipping the straps over my shoulders, I whisper an instruction to myself.

‘Whatever you do, do not fall over.’

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