Big Barry Freud was temporarily off the phone and looking exhausted when Bolt walked into his office.

'You know,' he said as Bolt sat down, 'even with all this bloody stuff going on, I've still got Thames Valley giving me crap about you driving off from the murder scene last night. I've had their assistant chief constable on the phone twice this morning. He sounds like a right old woman. He wants you interviewed in connection with their inquiry but I've told him you're not available at the moment. I won't be able to put it off much longer, though.' He paused in his monologue to wipe sweat from his brow with a handkerchief that looked like it had had a fair amount of use already that day.

'I've got a lead,' announced Bolt, and he told Big Barry about the dark blue Mazda Fallon had photographed at the Brakspear residence. 'If the kidnapper doesn't know that Fallon got a shot of the number plate, he might still be using it now. If we can find him, we might be able to find Hook and the gas.'

Big Barry grinned, seemingly pleased with this new information. 'Got to be worth a try, hasn't it? I'll get on to the ANPR people.'

The automatic number plate recognition system was the latest technological tool available to the police in the twenty-first-century fight against crime. It used a huge network of CCTV cameras which automatically read car number plates to log the movement of vehicles along virtually every main road in Britain. These images were then stored on a vast central database, housed alongside the Police National Computer HQ. If the Mazda had been driven in the past twenty-four hours, the ANPR would have a record of its journey.

Big Barry picked up the phone and two minutes later he was giving the Mazda's registration number to one of the senior officers in charge of the database, and telling him in no uncertain terms that his team could put everything else aside because tracing this car was the absolute number one priority. 'And that comes right from the very top, old mate,' he added, putting a faintly ludicrous emphasis on the word 'top'.

Big Barry Freud was the kind of man who liked to throw his not inconsiderable weight about, particularly during major inquiries. He believed that it was just part of his decisive take-charge personality, but to most other people, including Bolt, it was just plain rudeness.

Still, it seemed to work, and when he got off the phone he gave Bolt a decisive nod. 'He's going to call back in five minutes.'

'Any more progress on finding the lorry?' Bolt asked him. There were currently officers from three different police forces re-examining the camera footage from Harwich to see if they could identify it using just its physical description.

'Nothing yet,' said Big Barry. 'We must have two hundred bodies working on it, but Gould's wife hasn't been a lot of help. She says the lorry's big and white, with black writing down the side saying Banton Transport, which apparently isn't even that big. Oh, and that he's got a West Ham banner in the back of the cab, but she doesn't think you can see that very easily from the outside.'

'Shit. It's not a lot, is it?'

'No, it isn't. And you know what these CCTV images are like. They're blurry at the best of times. It's like the proverbial needle in the haystack, old mate.' He sighed. 'If we had some idea of what the target was going to be, it would help, but we haven't got a bloody clue.'

'Something like mustard gas is only going to be used for one thing: to cause mass casualties. Have we got any idea who Hook might be working for?'

Big Barry shook his head. 'Nothing. But I have had a briefing on the gas's properties and how it might be released. Apparently, if it gets ignited, mustard gas loses its potency, so they can't blow up the load with a conventional bomb. It's possible they can get someone with a decent gas mask to release it manually by opening up the cylinders one by one, but there are more than two hundred of them, so it would take ages, and as soon as people got a whiff of the first few they'd be off in no time, so it wouldn't be very effective.'

'So what are they going to do?'

'No one knows. But I've got a feeling they'll find a way.' He sighed again, and Bolt could see the pressure his boss was under. 'If they somehow get it out into the atmosphere, it'll be a bloody catastrophe. It's a sunny day with a light breeze, which is meant to be perfect conditions for releasing it. I don't mind telling you, I'm glad my missus isn't up in town today.'

Bolt was surprised at his honesty. Big Barry Freud usually towed the party line, but these, it seemed, were unprecedented times. 'Plenty of peoples' wives are,' he said, thinking about Mo and Saira, and their four children. He'd dropped Mo back at home on the way here earlier so he could grab a bit of sleep and they could spend some time together, and he wondered whether he'd sent them out of town as well.

'If the powers-that-be think there's a need to evacuate, then they'll do it,' said Big Barry, 'but they're setting up roadblocks coming into town and the congestion charge cameras are tracking any white lorries.' He was trying to sound confident but it wasn't really working, and he was saved from further conversation by the ringing of his phone.

It was clear the caller was from the ANPR. Barry wrote something down on the giant notepad that covered half his desk before hanging up.

'The Mazda was last caught on the ANPR yesterday afternoon at 2.47 p.m. just north of Saffron Walden in Essex on the B1052 at Linton in Cambridgeshire. If it's been used since, then it hasn't gone far because it would have been caught on one of the other cameras. They're going to send us a map showing the area where it might still be.'

Bolt smiled. 'That should narrow it down a bit.'

But when the map was emailed through to Barry's PC ten minutes later, it was clear that it hadn't narrowed things down as much as either of them would have liked. Although the Mazda's last location was surrounded by cameras, it was a largely semi-rural area of northern Essex, with hundreds of back roads and villages, bordered by the M11 to the west, and the computer-generated map calculated that the car could be anywhere in an area of almost 190 square miles.

'Christ, that doesn't help us much, does it?' said Big Barry as they pored over the printout. 'I've got a tag on those plates, so if the car starts moving again we'll know as soon as it's picked up by a camera. Until then, though…'

Bolt wasn't entirely deterred. 'You say the gas can't be released very easily manually, right? So, if they're going to come up with a way of releasing it effectively, they're going to have to take it somewhere to get it ready, don't you think?'

Big Barry shrugged. 'I don't know. It's possible, I suppose.'

'Well, maybe the car's gone to the same place. Hook hasn't got a big team. So far we only know of one person working with him.'

'I still don't see what you're getting at, Mike.'

'I'm thinking that maybe they've got a base round there somewhere. A centre for their operations.' He ran a finger in a wide circle on the map. 'A place they would have rented. If you can let me take a couple of people from my team off the CCTV trawl, we can look up all the estate agents in the area, see who's rented out property recently. It's a long shot…'

'An extremely long shot. We don't even know that the car hasn't just been abandoned in a wood somewhere.'

'But it's got to be worth a look. We've got two hundred people working on the CCTV. Surely we can spare a couple of them?'

Big Barry looked doubtful, but he was the sort of guy who always liked to cover all his bases, just in case there was an opportunity for personal advancement in one of them. 'OK, take one person.' He looked at his watch. 'It's eleven now. We'll review how you're getting on at two.'