Clambering to his feet, he saw that Mo had picked up Tina and was in the process of putting her over his shoulder. He rushed over and grabbed her legs, easing the load for Mo, and together they took off down the road, yelling at the two uniforms at the junction to do the same.

Mike Bolt had no idea how fast mustard gas travelled, but as he ran through the rain the adrenalin seemed to course through every part of his body. And though he knew his, Mo's and Tina's lives were in danger he had a bizarre yet incredibly intense desire to laugh out loud. He was enjoying this, he truly was. It was like all those dreams of action and adventure he'd had as a young kid. Now, aged forty and banging on the door of middle age, here he was running for his life with the heat of an exploded bomb at his back.

He and Mo ran with Tina for two, three, four hundred metres, it was difficult to tell. He felt a surge of relief when she moved a little and groaned, told her it was going to be OK, and kept going, knowing that if they made it out of this it had to be a good omen for all three of them.

But his legs were getting weaker, and he was slowing down badly. As was Mo, who was panting like an old man, two decades of cigarettes taking their toll. So it was with another burst of relief that Bolt saw the police van approaching slowly, its sirens flashing, and the man in the protective white suit leaning out of the passenger window and motioning for them to get in the back.

He pulled on the rear door handle and he and Mo threw Tina inside before being pulled in themselves by two uniforms.

'Is anyone else down there?' came a voice from the front.

Bolt thought of the thug he'd floored a few minutes earlier. 'No,' he gasped, 'I don't think so.'

Immediately the van turned round and they were driving out of there.

Still lying on the floor, he looked across at Tina. She was bruised, splattered in dried blood, and beautiful, her eyes just about staying open. She managed a weak smile. 'Thanks, Mike,' she whispered. Then her eyes closed.

Bolt smiled across at Mo, who was too busy getting his breath back to notice, then he reached over and took her hand, utterly elated that somehow she'd come out of this alive.

That they were all still alive.

Chief Superintendent Ken Canaver of Essex Police was standing on a grassy verge directly opposite the outbuilding he and his officers had been told was the possible headquarters of a terrorist cell, watching as flames gouting thick black smoke lit up the sky over to the west. He'd heard the dull thud of the explosion that had caused the fire and knew that it was the lorry his colleagues were trying to intercept. He also knew what it was supposed to contain. But he had no idea whether in the current weather conditions the gas would spread to where they were now, and until he heard otherwise he and his officers would remain where they were.

Canaver was a solid career copper, only one year short of his thirty years' service, and he liked to do things methodically and by the book, because he knew that, ultimately, that was the best way. In all his time in the police he'd never had to make a life-or-death decision, and he was truly hoping that this wasn't going to change now. As well as a fleet of ambulances, Canaver had some forty officers on the scene, a dozen of whom were armed. As he'd already announced to the building's occupants on the megaphone several times in the last ten minutes, he had the place surrounded. Neither the hostage negotiation team nor the big guns from Counter Terrorism Command and SOCA were yet at the scene, but the sooner they were, the happier he'd be. In the meantime he'd carry on repeating his request every three minutes for whoever was inside to give him or herself up. So far he'd received absolutely no response, although there were several lights on inside, so he and his people continued to stand silently in the pouring rain using a line of squad cars as cover, waiting to be relieved.

Behind him he heard several of the other officers whispering urgently to one another. The explosion had made everyone jumpy. Luckily, none of them knew its ramifications. The only people within the Essex police force who'd been informed that the lorry was carrying poison gas were the chief constable, his assistant, and Canaver himself.

Canaver fingered his mobile phone nervously, wondering if he was going to get a call to evacuate. As well as terrorists, he'd been told that the building might also contain a kidnap victim, although whether she was alive or not was still unclear. There was definitely someone alive inside though: two of his officers had seen movement in one of the upstairs windows a few minutes earlier. He didn't like the idea of abandoning a potential victim of crime, or letting the criminals holding her get away, but he had to admit that he'd be more than happy to leave this scene and its heavy responsibilities behind.

'I didn't expect an evening like this when I came on duty today,' said DCI Nigel Teasdale, the head of Essex CID and a colleague of Canaver's for more than ten years now.

The two men had never got along particularly well. Teasdale was brash, impulsive, and far too gung-ho – a trait that was definitely not needed in a siege situation – but right now it was all hands on deck and Canaver had no choice but to work with him.

'I don't think any of us did,' answered Canaver, wondering how Teasdale would react if he was told what the lorry contained. For all his bravado, the fat sod would probably run a mile, which given the size of his gut would be a sight worth seeing.

The thought momentarily cheered Canaver, but only momentarily, because as he stared straight ahead at the barn he saw smoke beginning to seep out of one of the windows on the upper floor, and the first flickering glow of flames coming from inside.

Others saw it too, including Teasdale. 'Blimey, he's burning the place down,' he announced loudly in a statement of the blindingly obvious. Then he asked the question Canaver had been dreading: 'What the hell do we do now?'

In the same moment, the head of the armed response team, Sergeant Tony Lennis, appeared at Canaver's other side. 'Do you want us to go in?' he asked.

The truth was that Canaver had no plan of action, no idea of the numbers he was up against or how well they were armed. Even the blueprints for the building hadn't arrived yet. Lennis might have been a firearms officer for close to two decades, but he'd never fired a shot in anger, and if he messed things up now it would be Canaver's responsibility.

The two men were looking at him expectantly. In the skies above, a helicopter circled noisily. Smoke was pouring out now, the flames rising higher. He could call the chief constable, put the onus on him, but that might look like indecisiveness, and time was running out. There could be someone in there in huge danger.

Christ, how he hated being put in this position.

He turned to Lennis, saw the pent-up tension in the man's face, the way he was bouncing up and down on the balls of his feet. 'All right,' he said, the words coming out with difficulty. 'Go in.'