He let out his breath. ‘You really shouldn’t do that,’ he said quietly, hoping to ease his tension. ‘You scared me half to death.’

He rubbed the back of his neck, and when his heart had stopped hammering and his legs shaking he stepped forward for a closer look.

It was a life-size painting of a man, wearing a Roman toga and standing imperiously with one arm resting on a marble column and the other stretching out towards the viewer.

His close-cropped hair was white and his chalky skin almost luminous. James felt uneasy looking at him; the eyes were painted in such a way that they appeared to be looking straight back at you.

At the bottom of the frame four letters were carved into the ornate wood – UCMM – but there were no other clues as to who the man might be.

‘I’ll see you later,’ said James and he quickly opened the door.

He found his way blocked by a heavy curtain. He pulled it aside and walked through. If the courtyard outside resembled a temple, this room appeared to be a chapel. There were bare walls and a plain wooden table set up like an altar at one end.

But this was no Christian chapel.

The painting above the table showed a man in Roman armour plunging a sword into a bull’s neck and sending a long spray of scarlet blood into the air.

There were two bowls on the table, ordinary cooking bowls, but one contained the head of a cockerel and the other was half full of a thick brown liquid that was congealing around the edges.

James sniffed it and remembered one of the Latin words he had recognised – sanguis, blood.

This wasn’t right. He was suddenly very scared and knew he had to get out of there quickly. He hurried to the door at the other end of the chapel and opened it. It led to a short flight of stairs. He tiptoed up and found himself back at ground level. There was a short corridor here with a shuttered window. He swung the shutters back and peered out. The street looked empty. As soundlessly as possible, he opened the window and swung out on to the ledge, then dropped to the pavement.

He was unsure of exactly where he was, so he went carefully, keeping to the shadows and heading in what he hoped was the general direction of Codrose’s. He was beginning to think that he was lost when he turned a corner and almost bumped into the ladder that the men had put up against the wall. There was still no sign of anyone, but, far off, he heard a shout and running feet and without a second’s thought he scaled the ladder two rungs at a time.

In a minute he was back up on the roof peering down through the dome in Codrose’s ceiling and he realised he’d only just made it in time; the fat man from the search party was there, talking excitedly to Codrose, who stood up, pulled on his jacket and left the room.