I turned back finally to the old parchment Cromwell had shown me in his room, the picture of the ship spouting Greek Fire, with the top part torn off. I ran my fingers along the torn edge. That act had cost Michael Gristwood his life.

'Better the monks had destroyed everything,' I whispered aloud.

I heard footsteps and looked up to see Barak approaching. He glanced over the flower beds.

'This is a fine-smelling place.' He nodded at the documents surrounding me. 'Well,' he said, 'what d'you make of it all?'

'Not much. For all this great jangle of words no one seems to have any clue what Greek Fire really was. As for the alchemical works, they are incomprehensible riddles and obscure words.'

Barak grinned. 'I tried to read a law book once, it made me feel like that.'

'Guy may be able to make some sense of them.'

'That old black monk of yours? He's well known round where I lodge. By God, he's a strange-looking one.'

'He's a very knowledgeable man.'

'Ay, so they say round the Old Barge.'

'That is where you live?' I remembered those shutters closing.

'Ay, it's not a fine place like this but it's in the middle of London – useful as my business takes me all over the City.' He sat beside me and gave me a sharp look. 'You're to say as little as possible to the black monk, remember.'

'I'll ask him to elucidate these alchemy books, say it's something I've to look into for a client. He won't press me more than that, he knows I have to keep clients' confidences.'

'Guy Malton, the black apothecary calls himself,' Barak said thoughtfully. 'I'll wager that's not the name he was born with.'

'No, he was born Mohammed Elakbar; his parents converted to Christianity after the fall of Granada. Your own name's unusual, come to that. Barak, it is like Baruch, one of the Old Testament names reformers are giving their children now. But you're too old for that.'

He laughed and stretched long legs in front of him. 'You're a scholar, aren't you! My father's family was descended from Jews who converted to Christianity in old times. Before they were all kicked out of England. I think of it whenever I have to visit my master at the Domus. So maybe it was Baruch once. I've a funny little gold box my father left me that he said had been passed down from those days. It was all he had to leave me, poor old arsehole.' Again that sombre look passed quickly across his face. He shrugged. 'Anything else those old papers reveal?'

'No. Except I think the monks hid the formula and the barrel for fear of the destruction Greek Fire could cause.' I looked at him. 'They were right. The devastation such a weapon could wreak would be terrible.'

He returned my look. 'But if it could save England from invasion. Surely anything is worth that.'

I did not reply. 'Tell me what it was like. At the demonstration.'

'I will, but tomorrow at the wharf. I came to tell you I'm going out. I have to fetch some clothes from the Old Barge. And I am going to ask around the taverns, see if any of my contacts know of that pock-faced man. Then afterwards I've a girl to see, so I'll be back late. Got a key?'

I looked at him disapprovingly. 'Ask Joan for hers. We must start very early tomorrow.'

He smiled at my look. 'Don't worry, you won't find me wanting in diligence.'

'I hope not.'

'Nor will the girl.' He gave me a lubricious wink and turned away.

THAT NIGHT I COULD NOT SLEEP, from the heat and from the tangle of images that chased each other through my mind: Elizabeth in her cell, Cromwell's drawn, anxious look, that pair of dreadful corpses. Far into the night I heard Barak come in, footsteps creeping quietly upstairs to his room. I rose and knelt by the bed in the sticky darkness to pray for rest and guidance on the morrow. I was praying less and less these days, feeling often that my words did not ascend to God but merely dissolved inside my head like smoke, but when I returned to bed I fell at once into slumber and woke with a start to the light of early morning, a warm breeze wafting through the open window and Joan calling me down to breakfast.

Despite his night of rousting, Barak seemed fresh as a new pin, eager to be off. He told me he had been unable to trace the man who had followed us, but had set enquiries in train among his acquaintances. Straight after breakfast we walked down to catch a boat at Temple Stairs. It was not yet seven; I was seldom abroad at such an hour on a Sunday and it was strange to see everywhere deserted. The river, too, was quiet, the wherrymen waiting idly at the stairs pleased to have our business. The tide was at low ebb and we had to walk to the boat across a wooden catwalk laid over the thick, rubbish-strewn mud. I turned my head from the smell given off by the bloated carcass of a dead donkey. I was glad to step into the boat. The wherryman steered us into the middle of the river.

'D'you want to shoot the rapids under London Bridge?' he asked. 'It'll be an extra half-groat.' He was an ill-favoured young fellow with the scar of some old fight running down his face; the Thames boatmen were ever a battlesome crew. I hesitated, but Barak nodded. 'Ay, the water's at its lowest, there won't be much pull under the piers.'

I gripped the sides of the boat as the great bridge, crowded with houses, loomed up, but the wherryman steered us deftly through and we floated on downriver past Billingsgate, where the big seagoing ships lay docked, past the looming mass of the Tower of London. Then we passed the new naval docks at Deptford, and I stared in wonder at the king's great warship Mary Rose, in for repair, her enormous masts and rigging soaring high as steeples above the surrounding buildings.

Beyond Deptford signs of habitation ended and the river broadened, the far bank growing distant to the view. Wastes of marsh and reeds crowded to the water's edge. The occasional wharves we passed were mostly abandoned, for shipworking was concentrated upriver now.

'That's it,' Barak said at length, leaning over the side. A little way off I saw a crumbling jetty rising on wooden piers. Behind, a space of weed-strewn earth cleared from the surrounding reed beds fronted a large, tumbledown wooden shed.

'I expected something larger,' I said.

'My master chose it because it was out of the way.'

The wherryman guided the little boat to the jetty, grasping at a ladder fixed to the end. Barak climbed nimbly up. I followed more carefully.

'Come back for us in an hour,' Barak told the boatman, passing him his fare. He nodded and cast off, leaving us alone. I looked round. Everything was silent and still, the surrounding reeds whispering in the light breeze, richly coloured butterflies flitting among them.

'I'll just check the shed,' Barak said, 'in case some vagabond has made a home there.'

As he went to peer through the warped boards of the shed, something dangling from a ring in an iron bollard caught my eye. A thick, knotted hemp rope, such as might be used to tie up a boat, hung over the end of the jetty. I drew it up. There were only about two feet of rope; the end was charred. It had been burnt right through.

Barak rejoined me. 'All clear.' He passed me a leathern bottle. 'A drink?'

'Thank you.' I unstoppered it and took a draught of small beer. Barak nodded at the rope which I still held. 'That's all that's left of the boat I tied up there.'

'Tell me,' I said quietly.

He led me into a patch of shade cast by the shed. He looked out over the river for a moment, then took another draught of beer and began his tale. He told the story with more fluency than I would have expected, a sense of wonder overcoming his usual brashness.

'Back in March my master told me to buy an old crayer, in my own name, and have it brought down here. I found one, a big thirty-foot tub, and had it rowed down and moored here.'