He tore open the plastic bags and tipped most of the ice into the shower and then fetched a box of salt and sprinkled it over the cubes before pushing the bottles of acid into the freezing mixture.

While it cooled he emptied the holdall on to the bed. There was the bottle of glycerine, a can of motor oil, several boxes of matches, a tube of glue, a jumble of plastic piping, plastic-coated wire, a box of baking soda, a pair of washing-up gloves, a thermometer, a Pyrex measuring jug, two large Pyrex saucepans and a Teflon-covered stirrer. He took what he needed into the bathroom and switched on the light so that the ventilator would start working. The acid fumes would be painful if he inhaled them.

Nguyen had worked with explosives a lot during the war. Whenever possible the Vietcong had bought explosives or used equipment captured from the Americans, but supplies weren’t always easy to get and they were quite capable of manufacturing their own blasting gelatine, TNT, plastic or nitroglycerine. Most of the raw materials for explosives could be bought quite legally, though in the later years of the war there were restrictions on the sale of electric timing devices. Not that it mattered, though, because clockwork alarm clocks were just as good.

It took almost half an hour before Nguyen had completed the complex and dangerous series of chemical reactions that left him with an oily white substance forming a milky layer at the bottom of the measuring jug.

Nguyen settled back on the cold floor and sighed. His jaw ached and he realised he must have been grinding his teeth with the tension. He was starting to get a headache, a piercing pain behind his eyes that could be a result of the stress or more likely the effect of the fumes. He got to his feet, his knees cracking as he straightened up and walked unsteadily into the bedroom. He opened the window wide and then sat on the edge of the bed, breathing deeply to clear his head.

When he felt a little better he took the bag of fertilizer into the bathroom and tore it open. He spread out one of the empty plastic bags that had contained the ice and scooped handfuls of fertilizer on to it. it was important to get the ratio of motor oil, nitroglycerine and fertilizer right. He added the oil to the fertilizer first, kneading it like dough until it had been absorbed and then carefully poured out the nitroglycerine a little at a time, placing it back in the shower between pourings. The nitroglycerine could be used as an explosive on its own but it was dangerously unstable and would explode if knocked or dropped or if it got too hot. Once it had been mixed with the fertilizer and oil it would be quite inert until detonated but would be almost as effective.

Nguyen worked slowly and methodically and it took him the best part of an hour until all the nitroglycerine had been worked into the mixture and he had a dark-brown gooey paste on the plastic bag. He stripped off the gloves and laid them on the floor and went back into the bedroom. The headache was worse. He looked at his watch. It was six o’clock. He had plenty of time, so he lay down on the bed and rested.