The hotels yielded nothing. The few bed-and-breakfast houses open for business in January reported only sales reps as overnight guests.

‘Check them again, and every day from now till Friday,’ Martin ordered.

Skinner briefed the team on Tuesday morning on the discovery in Mortimer’s files.

Mackie looked embarrassed. ‘Sorry, boss,’ he said. ‘I should have found that.’

‘Bollocks,’ said Skinner dismissively. ‘You’re a copper not a computer man.’

To keep the team active, Martin sent them out to make their second round of checks in person, rather than by telephone. ‘Remember that cover story. We’re trying to trace him because of trouble at home.’

After the four had left, Skinner picked up his jacket, and motioned to Martin to follow.

‘Where are we going?’

‘We’re going to the airport to meet a man. We wear these so he’ll recognise us.’ He pinned a small gold lion badge into his lapel, and handed one to Martin, who fastened it to his tie. The lions were sometimes used by Special Branch and protection officers to indicate to each other that the wearer was armed.

The visitor approached them quietly as they stood at the bookstall opposite the British Midland arrival point.

‘Mr Skinner, Mr Martin? I’m Maitland.’ He spoke in flat clipped tones, with no trace of a regional accent.

The man stood just over six feet tall. He might have been around thirty years old. He was clean-shaven, and his dark hair was close-cropped. His eyes were blue, as clear as a bell, and he wore the fading tan of someone recently returned from a spell in a seriously hot place. He wore a well-tailored, double-breasted suit of navy-blue worsted, with a thin vertical stripe.

He did not give the impression of physical power, but when the two policemen shook his hand they found a grip like a vice. His carriage was his most impressive feature. He walked out of the terminal building, between Skinner and Martin, with lightness, grace and perfect balance, as if his feet were hardly touching the ground.

Maitland had introduced himself in a confidential fax to Skinner as the commander of the Special Air Services detachment which had been assigned to provide cover for the Syrian President during his visit. He had not mentioned his rank, but Skinner knew that in the SAS, that was not important.

Martin drove to the Norton House, where the three were met by the manager, an immaculate man named Adrian Doyle. Skinner described Maitland as ‘a security adviser who will be here during the visit’. Doyle, who had previous experience of VIPs, asked no questions.

He guided them round the hotel. In the first-floor suite which had been set aside for the Syrian President, Maitland made a careful check of the angles of view through the double window as they related to the position of the main items of furniture. He opened a window and checked for drainpipes or other climbing aids, and found nothing. Leaning further out, he surveyed the roof above. He confirmed that there were no points of access to the en suite bathroom, other than the door from the bedroom.

Eventually he turned to the expectant Doyle. ‘It looks secure, but I’d like you to move the bed to that wall. We legislate for everything, even the sort of fanatic who will empty a magazine through a curtained window if he can’t find a better opportunity. At the moment the bed is in the line of fire from those trees over there.’

Doyle smiled. ‘There will be no difficulty about that.’ He took them back to the entrance hall and left them to explore the hotel grounds alone.

The grass and trees were wet from the previous night’s rain, but Maitland was prepared. He produced a nylon coverall from his bag in Skinner’s car, discarding his jacket before slipping it on. His black leather shoes were replaced by trainers.

‘No need to come with me, gentlemen. All I’ll be doing is checking the terrain, and identifying all the possible firing points.’ the disappeared into the woods.

When he emerged silently behind Skinner and Martin fifteen minutes later, the coverall was dripping wet.

‘You’ve made a very good choice,’ he said, as he stepped out of the garment. ‘I will have twenty men here. With that number, I could keep a fly out of this place.

‘When are the technical people installing the listening devices and cameras?’

‘Thursday,’ Martin replied.

‘Good. I’ll advise them on siting the video cameras. My men arrive on Thursday too. I’d like to do a rehearsal of the whole operation that evening, including the Hall. Can we check that out now?’

Their visit to the MacEwan Hall was quickly concluded. Henry Wills was there to greet Maitland, but he left as soon as the welcome was over, with what Martin read as a tiny shudder of distaste for the man and his business.

The SAS leader checked the outside of the building for entry points. Then he inspected all the doorways leading into the Hall itself.

‘Piece of cake. You clear the building a few hours in advance and the specialists do the bomb search. No admission until an hour before the kick-off. Everyone entering is frisked, and all bags are searched. But no metal detectors.’

Martin was surprised. ‘Why not?’

‘This is a student audience. They’ll be wearing all sorts of odds and ends. Big belt buckles, bracelets, all sorts of stuff that would set the alarms ringing. We’d never get them all in in time.

‘You put four good people here doing thorough body and bag searches. If anyone tries to smuggle a gun in they’ll find it.

‘My unit will cover this place easily. We’ll cover all entrances to the building, and doorways to the Hall itself. None of the students will know we’re there. Even you won’t notice us.’

They drove Maitland to Redford Barracks, on Edinburgh’s southern outskirts, where he and his men were to be billeted. As Maitland jogged the few yards from the car into the long imposing building, Skinner looked after him for several seconds.

‘That, Andy, is probably one of the most dangerous men you will ever meet.’

Suddenly Martin was aware of his own lack of experience. He began to understand the reason for Henry Wills’s quick exit.