‘Oh yes.’ His face broke into a smile. ‘Hello, old friend.’

‘You recognise it?’

‘Wouldn’t you recognise one of your own children?’ Ziff asked impatiently. ‘Especially one as special as this.’

‘What do you mean?’ Tom shot back eagerly.

‘Each of my watches is normally unique,’ Ziff explained. ‘A one-off. But in this case, the client ordered six identical pieces. And paid handsomely for the privilege, from what I remember.’

‘Six?’ Allegra repeated excitedly. They knew of four already. That left two others still unaccounted for.

‘They’re numbered,’ Ziff continued, pointing at the delta symbol delicately engraved on the back of the case. ‘Platinum bezel, stainless-steel case, ivory face, self-winding, water resistant to thirty metres, screw-down crown…’ He balanced it in his hand as if weighing it. ‘A good watch.’

‘Who was the client?’ Tom asked.

Ziff looked at him with an indulgent smile, slipping his glasses up on to his forehead where they perched like headlights.

‘Felix, you know better than that.’

‘It’s important,’ Tom insisted.

‘My clients pay for their confidentiality, the same as yours,’ Ziff insisted with a shrug.

‘Please, Max,’ Tom pleaded. ‘I have to know. Give me something.’

Ziff paused before answering, his eyes blinking, then slipped his glasses back on to his nose and stood up.

‘Do you like pinball?’

‘We’re not here to play pinball,’ Tom said sharply, although Ziff didn’t seem to pick up on his tone. ‘We’re here to…’

‘“Straight Flush” is a classic,’ he interrupted, crossing over to the door. ‘Why don’t you have a game while you’re waiting?’

‘Waiting for what?’ Tom called after him, but Ziff was already out of the room, the sprung door easing itself shut behind him.

Allegra turned towards the machine he had pointed out. It appeared to be one of the oldest and most basic in the room, the salmon-coloured back-board illustrated with face-card caricatures, the sloping yellow surface decorated with playing cards that Allegra guessed you had to try and illuminate to create a high-scoring poker hand. She frowned. It wasn’t an obvious recommendation, compared to some of the more modern, more exciting games in the room, but then again she had detected an insistent tone in his voice. A tone that had made her wonder if there was something there he wanted them to see. Something other than the machine itself.

‘Can you open it?’ she asked, pointing at the metal panel on the front of the machine that contained the coin slot.

‘Of course,’ Tom squatted down next to her with a puzzled frown, reaching into his coat for a small pouch of lock-picking tools.

‘He said that each machine was for a job he’d completed. A tombstone so he wouldn’t forget,’ she reminded him as he deftly released the lock and opened the door, allowing her to reach into the void under the playing surface. ‘I just wondered…’

Her voice broke off as her fingers closed on an envelope of some sort. Pulling it out, she opened it, the flap coming away easily where the glue had dried over the years. It contained several sheets of paper.

‘It’s the original invoice,’ she exclaimed with an excited smile. ‘Six watches. Three hundred thousand dollars,’ she read from the fading type. ‘A lot of money, thirty years ago.’

‘A lot of money today.’ Tom smiled. ‘Who was the client?’

‘See for yourself.’

Allegra handed him the sheet, her eyes blazing with excitement.

‘E. Faulks & Co,’ Tom read, his face set with a grim smile. ‘And there’s a billing address down at the Freeport. Good. I’ll ask Archie to meet us there. Even if Faulks has moved we should be able to find -’

‘That’s strange,’ Allegra interrupted him, having quickly leafed through the rest of the contents of the envelope. ‘There’s another invoice here. Same address, only twelve years later.’

‘But that would make seven watches.’ Tom frowned. ‘Ziff only mentioned six.’

Before she could even attempt an explanation, she heard the whistled strains of the overture from Carmen echoing along the corridor outside. Snatching the invoice from Tom’s hand, she slipped it back in the envelope, shoved it inside the machine and shut the door.

“Magnets,’ Ziff announced as he sauntered in, excitedly waving several sheets of paper over his head. ‘I knew they were down there somewhere.’

‘What?’

‘Magnets,’ Ziff repeated with a high-pitched giggle, his glasses hanging around his neck like a swimmer’s goggles. ‘See.’

Picking D’Arcy’s watch up, he held it over the tray containing the watch he was working on. Two small screws leapt through the air and glued themselves to the bezel.

‘Each watch has a small electro magnet built into it powered by the self-winding mechanism,’ he explained, opening the file and pointing at a set of technical drawings as if they might mean something to either of them. ‘They were all set at slightly different resistances.’

‘What for?’

‘Some sort of a locking mechanism, I think. They never said exactly what.’

Allegra swapped a meaningful glance with Tom. So this was why Santos needed the watches. Together, they formed a key that opened wherever the Caravaggio was being stored.

‘Normally I destroy the drawings once a job is completed, but this was the first time I had used silicon-based parts and I thought they might be useful. Turns out it was just as well.’

‘What do you mean?’

‘The client lost one of the watches and asked for a replacement. The epsilon watch, I think. Without these I might have struggled to replicate it.’

Allegra took a deep breath. That explained the second invoice. More importantly, it meant that there were seven numbered watches out there somewhere. Each the same and yet subtly different. Each presumably entrusted to a different key member of the Delian League.

‘By the way, what was your score?’ Ziff jerked his head towards the ‘Straight Flush’ pinball machine he had pointed out earlier.

‘Ask us tomorrow,’ Tom answered with a smile.

Free Port Compound, Geneva 20th March – 12.02 p.m.

The Free Port was a sprawling agglomeration of low-slung warehouses lurking in the shadow of the airport’s perimeter fence. Built up over the years, it offered a vivid snapshot of changing architectural fashions, the older buildings cinder grey and forbidding in their monolithic functionality, the newer ones iPod white and airy.

For the most part, its business was entirely legitimate, the facilities providing importers and exporters with a tax-free holding area through which goods could be shipped in transit or stored, with duty only being paid when items officially ‘entered’ the country.

The problem, as Tom was explaining to Allegra on the drive down there, lay in the Free Port’s insistence on operating under a similar code of secrecy to the Swiss banking sector. This allowed cargo to be shipped into Switzerland, sold on, and then exported again with only the most cursory official records kept of what was actually being sold or who it was being sold to. Compounding this was Switzerland’s repeated refusal to sign up to the 1970 Unesco Convention on the illicit trade in cultural property. Not to mention the fact that, under Swiss law, stolen goods acquired in good faith became the legal property of the new owner after five years on Swiss soil.

Taken together these three factors had, over the years, established Switzerland’s free ports as a smuggler’s paradise, with disreputable dealers exploiting the system by secretly importing stolen art or looted antiquities, holding them in storage for five years, and then claiming legal ownership.

To their credit, the Swiss government had recently bowed to international pressure and both ratified the Unesco Convention and changed its antiquated ownership laws. But so far the Free Port’s entrenched position at the crossroads of the trade in illicit art and antiques seemed to be holding surprisingly firm. As Faulks’s continuing presence served to prove.