All around, the great vaulted arches of the nave rose nearly a hundred feet, supported by pillars brightly painted in red and black. Blue and yellow tiles covered the floor. The eye was led to the high stone rood screen halfway down the nave, richly painted with figures of the saints. On top of the screen, lit by candles, stood the statues of John the Baptist, the Virgin and Our Lord. A great window at the far end of the church, built to catch the morning light from the east, was painted in geometric designs of yellow and orange. It flooded the nave with a gentle umber light, peaceful and numinous, softening the kaleidoscope of colours. The builders knew how to create atmosphere, no doubt of that.

I walked slowly up the nave. The walls were lined with painted statues of saints and little reliquaries, where strange objects peeped out from beds of satin, candles burning before them. A servant moved slowly around, replacing those that had burned down. I paused to glance into the side chapels, each with its own statues and little candlelit altar. It occurred to me that these side chapels, filled with railed-off altars, statues and biers, might be good places to hide things.

In several of the side chapels monks stood intoning private Masses. Local people of wealth, terrified of the pains of purgatory awaiting them, would have left great portions of their assets away from wives and children to the monks, for Masses to be said until the Last Judgement came. How many days' remission from purgatory was a Mass here worth, I wondered; sometimes a hundred were promised, sometimes a thousand. While those without means were left to suffer for however long God appointed, of course. Pick-penny purgatory, we reformers called it. The Latin chanting stirred an impatient anger in me.

At the rood screen I stopped and looked up. My breath, still a fog, for the church was scarcely warmer than outside, dissipated into the yellow-tinted air. On either side a flight of stairs set into the wall gave access to the top of the screen. At that level, I saw, a narrow railed parapet ran the length of the church. Above the parapet the walls arched gradually inward to the great vault of the roof. To the left I noticed a great crack, stained round with damp, running from the roof almost to ground level. I remembered that Norman churches and cathedrals were not in fact the solid edifices they appeared; the walls might be twenty feet thick, but between the expensive stone blocks making up the interior and exterior walls there was usually an infill of rubble.

Where the abscission ran down the wall the stone blocks, and the plaster between them, were discoloured and there was a little heap of powdery plaster on the floor beneath. I saw that above the parapet a series of statues were set in niches at intervals; they showed the same figure of St Donatus leaning over the dead man that was on the monastery seal.

Where the crack ran through one of the niches the statue had been removed and lay, discoloured-looking, on the parapet. An extraordinary cat's cradle of pulleys and ropes had been set up there; the ropes were secured to the wall behind the parapet and ran out over the void, before disappearing upward into the darkness of the bell tower, where presumably they were secured at their other end.

Dangling from the ropes was a wooden basket, big enough to hold two men. Presumably the cat's cradle allowed the basket to be moved inwards and outwards and had allowed the removal of the statue. It was an ingenious arrangement but a dangerous one; scaffolding was surely needed to effect proper repairs. But the bursar was right to say a full repair programme would be enormously expensive. Otherwise, though, as frost and water did their work, the crack could only widen, eventually threatening the whole structure. The imagination reeled at the thought of the great building falling on one's head.

Apart from the susurration of prayers from the side chapels, the church was silent. Then I caught a faint murmur of voices, and followed the sound to where a little door stood ajar, candlelight flickering within. I recognized the deep voice of Brother Gabriel.

'I've every right to ask after him,' he was saying in angry tones.

'If ye're always round the infirmary, people will be talking again,' the prior replied in his harsh voice. A moment later he emerged, his ruddy face set hard. He started a little when he saw me.

'I was looking for the sacrist. I thought he might show me the church.'

The prior nodded at the open door. 'Ye'll find Brother Gabriel in there, sir. He'll be glad to be taken from his desk in this cold. Good morning.' He bowed quickly and passed on, his footsteps echoing loudly away.

The sacrist sat behind a table strewn with sheets of music in a little book-filled office. A statue of the Virgin leaned drunkenly against one wall, her nose broken off, giving the bitterly cold, windowless room a depressing air. Brother Gabriel sat at a table, a heavy cloak over his black habit. His lined face was anxious; in some ways it was a strong face, long and bony, but the mouth was pulled down tightly at the corners and there were deep bags beneath his eyes. At the sight of me he rose, forcing his mouth into a smile.

'Commissioner. Master Shardlake. How may I help you?'

'I thought you might show me the church, Brother Sacrist, and the scene of the desecration.'

'If you wish, sir.' His tone was reluctant, but he stood and led me back into the body of the church.

'You are responsible for the music, Brother, as well as the upkeep of the church?'

'Yes, and our library. I can show you that too if you wish.'

'Thank you. I understand Novice Whelplay used to help you with the music.'

'Before he was sent to freeze in the stables,' Brother Gabriel said bitterly. Collecting himself, he continued in a milder tone. 'He is very talented, though rather over-enthusiastic.' He turned anxious eyes on me. 'Forgive me, but you are lodging in the infirmary. Do you know how it goes with him?'

'Brother Guy believes he should recover.'

'Thank God. Poor silly lad.' He crossed himself.

As he led me on a circuit of the church he became a little more cheerful, talking animatedly about the history of this or that statue, the architecture of the building and the workmanship of the stained-glass windows. He appeared to find a refuge from his anxieties in words; it seemed not to strike him that as a reformer I might not approve of the things he was showing me. My impression of a naive, unworldly man was reinforced. But such people could be fanatical, and I noticed again that he was a big man, strongly built. He had long delicate fingers, but also thick strong wrists that could easily wield a sword.

'Have you always been a monk?' I asked him.

'I was professed at nineteen. I have known no other life. Nor would I wish to.'

He paused before a large niche containing an empty stone pedestal, on which a black cloth had been laid. Against it was heaped an enormous pile of sticks, crutches and other supports used by cripples; I saw a heavy neck-brace such as crookback children wear to try and straighten them; I had worn one myself, though it did no good.

Brother Gabriel sighed. 'This is where the hand of the Penitent Thief stood. It is a terrible loss; it has cured many unfortunate people.' He gave the inevitable glance at my back as he spoke, then looked away and gestured at the pile.

'All those things were left by people cured by the Penitent Thief's intervention over the years. They no longer needed them and left them behind in gratitude.'

'How long had the relic been here?'

'It came from France with the monks who founded St Donatus's in 1087. It had been in France for centuries, and at Rome for centuries before that.'

'The casket was valuable, I believe. Gold set with emeralds.'