‘If it was someone who knew what they were doing, no more than a few minutes. There’s nothing clever about the device itself. It’s the method of detonation that’s a bit smarter than usual. Whoever attached the explosive must have worked out his method pretty well beforehand. It’s not something you’d be able to improvise on the spot. So, if you want my opinion, it wasn’t just some opportunist assassin slapping a bit of Semtex under the chassis when no one was looking.’

‘They needed access to the interior of the vehicle?’

‘Oh, yes.’

‘And they had to know in advance what they were dealing with?’

‘Exactly.’ The captain smiled. ‘Personally, I think that should make your job a lot easier. There can’t be many people around here with that sort of expertise.’

‘Well, we hope not.’

‘We’ve all heard such a lot about Semtex,’ said Hitchens. ‘I suppose it must be easy to get hold of if you know the right people?’

‘It used to be. Especially if you had contacts in Libya, which is where most of the stuff went to. It became the terrorists’ explosive of choice, because it was difficult to detect, easily obtained – and as little as two hundred and fifty grams could bring down an airliner.’

‘Two hundred and fifty?’

‘Slightly over half a pound. Think of a packet of butter.’

‘Are you sure? It doesn’t seem enough to bring down an airliner.’

‘Ask the people on board Pan Am 103, or the residents of Lockerbie. The device in that case was estimated to contain three hundred and twelve grams. It made a pretty thorough job, wouldn’t you say?’

‘You said it used to be easy to get hold of,’ said Hitchens. ‘Only used to be?’

‘For the last four years, all sales of Semtex have been under the control of the Czech government. They’ve added ethylene glycol dinitrate to produce a distinctive odour and aid detection. They’ve also tried to reduce the shelf-life; all new supplies contain an identifying code.’

‘So Semtex is more difficult to obtain now, and easier to detect?’

‘If it’s newly manufactured. The trouble is, there’s still a lot of the old stuff around. The security services reckon the IRA has about ten tonnes of it, for a start. They’d probably be willing to share it around if they’re paid enough.’

‘Could this be the sort of explosive used by quarrying companies?’

The expert looked at him pityingly.

‘Well, there are a lot of quarries around here,’ said Hitchens. ‘There are blasting operations going on all the time. If they were using this stuff –’

‘I doubt it. Plastic explosives are much more expensive than other materials that perform just as well for ordinary blasting. On the other hand, if you have a major demolition project in the area, that might be a different matter.’