I turned again and bumped into another wall. The hole was little bigger than a coffin. I was about to climb out but then felt Monk's hand on my arm and heard boots-what sounded like an army of them-thundering overhead. The intruders had reached the top of the stairs. A voice shouted my name again. I fumbled for a stool, somewhere to sit. I couldn't breathe. More stomping. Doors banging shut. I was going to faint…

But I did not faint. Monk slid a stool towards me, I seated myself, then for the next few hours the two of us listened to the commotions above us, faces upturned to the invisible hatch, frozen in silence as the intruders-three men, possibly four-opened doors and tapped every inch of the house with their swords and sticks. Our guests were very thorough. The staircase, the stone jambs of the fireplace, its mantelpiece and hearthstone, the ceilings and floors, the cupboards, wainscots, beds, curtains, every crumbling brick or worm-holed timber-nothing in the house was left untouched. Three times we heard them directly above us, thumping about in the corridor outside the boot-cupboard, then opening its door and tapping at its walls. But three times the cupboard door slammed shut and the footsteps and tapping receded. A moment later I heard soft blows a few bare inches from my ears as the end of a stick carefully sounded the wall of my study. But the partition was thick, filled with hair-plaster and pug, and the hollow sound, if there was one, must have been deadened. After a moment the tapping stopped. I expelled a sigh of relief and felt Monk's hand squeeze my shoulder.

'All right, sir?'

'Yes,' I stammered, a little too loudly. 'All right.'

I was trembling badly and hoped he couldn't tell, but I supposed it no longer mattered. Throughout our ordeal it seemed as if the roles of master and apprentice had been exchanged. From the first moment of our hasty escape up the staircase he had been patient and courageous, while I, his master, was nothing but terror, confusion and, later, complaints. I chafed terribly under the confinement. After only a few minutes on the stool my back ached; then my legs grew stiff and, a short time later, I realised that my bladder desperately needed relief. Then I couldn't breathe the thickening air. My chest gurgled, my diaphragm twitched and heaved as I stifled my basset-hound coughs, any one of which would have betrayed us. I bit my lip and tried to draw strength and comfort from the thought of the priest who had preceded us inside the cell, perhaps in circumstances like these, a little man kissing his Agnus Dei, telling his beads, reciting the Litanies of the Saints under his breath. But it was all I could do to keep from whimpering.

Yet Monk was in his element in the cramped, pitch-dark cell. It was as if he had long been preparing for this moment, or as if his earlier experiences with intruders had been a sort of crucible, making him patient and wise, no longer my obedient subaltern but an efficient, decisive leader, capable of planning and assessing. He was the one who decided that we could not afford a candle, who found the blanket to cushion my back, who whispered reassurances about our supply of air and chances of escape… and who, after the outside door banged noisily shut and everything fell silent, was able to tell that one man was still inside the house, standing perfectly still, waiting for us to emerge, which I had been only too anxious to do. A few minutes later, of course, we heard a low cough from inside the study. So we waited another couple of hours until he too had departed. Then Monk made a step with his interlaced fingers and hoisted me upwards. I clambered into the boot-cupboard, gasping for air and then emerging into the dawn-lit corridors and chambers like a survivor crawling from the rubble of a disaster.

Only there were no signs of disaster either in the house or the shop below. Certainly nothing like what had happened a few days before. We tiptoed through the rooms in semidarkness, keeping away from the windows-another of Monk's wise recommendations-and looking for any signs of what had happened. But it was as if no one else had been inside the house; as if the past few hours had been nothing but a shared nightmare. I even discovered the firelock on the bottom step, apparently untouched. The only evidence of our visitors was a faint whiff of torch smoke added to the fug of the house.

'Who d'you reckon they was, sir?' Up here, pacing the familiar corridors, Monk had reverted to being my deferential apprentice. 'Same coves as before, d'you think?'

'No, I think not.' We were inside the shop now, poking about with an eye on the green door. 'They weren't after our books, were they? Not like the men the other night.'

He nodded his head, and for a moment we gazed about in silence. No, none of the books had been touched. They still stood in the perfect ranks into which we had assembled them on their shelves only a few hours earlier. Nor had the men come for our money. The lock on the iron chest under my bed was untouched, as was the pouch of coins behind the shop's counter and, more importantly, the store of sovereigns and papers under the floorboards. Not so much as a tin farthing was missing from the house. I became aware that Monk's baffled, querying gaze had come to rest on my face.

'You reckon they came looking for you, then?'

I shrugged, unable to meet his prising glance. I turned round to inspect the lock on the door, which was intact, like everything else. The cracksmen, whoever they were, had known their business.

But just then something beside the door, a smudge of dirt, caught my eye, and I knelt to examine it. A dot of grey powder, a fairy-dust that was gritty to the touch and faintly iridescent in the morning light.

'What is it, sir?' Monk was leaning over my shoulder.

'Coquina,' I told him after a moment's inspection. 'Lime stone.'

'Limestone?' He was scratching his head and breathing audibly. 'From a quarry?'

'No, not a quarry. The sea. See this?' I blew on the powder to expose a tiny fragment, what looked like a bone chip. 'It's made from crushed cockle-shells.'

He ran a finger over the dust. 'Blimey, sir. How'd cockle shells get in here? You reckon they was brought inside by…?'

'I do indeed.' I straightened, still examining the fine chips in my palm. 'Coquina is used in road-making,' I explained. 'Carriageways in front of mansions, that sort of thing. It must have been tracked inside on their boots.'

Monk nodded solemnly as if waiting for me to explain something further, which I didn't. After a minute I brushed the dust from my palms and stood before the shuttered window. It was almost eight o'clock, by now. I watched through the louvres as the morning sunlight striped the floor behind me and etched long shadows on to the carriageway. The bands of light hurt my eyes and sent sharp pains radiating to the back of my skull. But I leaned forward and-just as I had done a half-dozen times in the past two days-peered up and down the lengths of carriageway. It was filling with morning traffic, with its familiar cacophony of shouting voices, ringing horseshoes, the iron clanking of bolts and bars as shops opened along the bridge. Apprentices with broomsticks materialised before them and swept at patches of sunlight.

I felt a painful throb beneath my breastbone as I watched the scene unfurl. This was my favourite moment of the day, the time when I would swing open the shutters, lower the awning, beeswax the counter and bookcases, cleanse the grate, light a fire, then bring a kettle of water to the boil for the first coffee of the morning and retire behind the counter and wait for my first customers to open the green door and step inside. But this morning I suspected the ritual would never be the same again. For who else, I wondered, might appear on the bridge this morning and then push inside the shop? Who else was out there, what evil eminence with his secret powers, hiding in the porches and doorways, watching the green door and waiting for the next time? Because what I hadn't told Monk was that the carriageways and footpaths of Whitehall Palace were covered with coquina-it had crunched under my feet as I wove my way to the offices of the Exchequer.