Got him, thought Bryant.

'Five minutes at the most,' he replied.

'I suppose if it appeared that there was a line fault with the system, I could have the entry circuits and the motion detectors switched off for two or three minutes, but no more than that. I don't know what I'd tell Albert.'

'Send him off to make tea or something.'

'The closed circuit monitors would have to be shut down, too. This boy has to find a letter, you say?'

'That's right.'

'Do you know which gallery it's in?'

Masters reclaimed the receiver. 'Perhaps you could help with that. We're looking for a painting in the Hever exhibition that features lots of crucifixes. There may be some kind of Chaucerian quote on the frame.'

'God, Harold, do you have any idea how many paintings there are in here? Your lad would have to know exactly where he was going. There'd be no time to mess about. If I know which gallery he's heading for I'll only have to disarm one sector, and it'll look less suspicious.'

'Tell him we'll work out where Vince has to go by the time he gets there,' hissed Maggie, tugging Masters's sleeve. 'There's less than thirty minutes left.' She looked questioningly at the rest of the group. 'Well, we can work it out in that time, can't we?'

THE ENTRANCE to the National Portrait Gallery was in darkness as Vince crossed Charing Cross Road by the rain-darkened statue of Edith Cavell, and the rotund little man who suddenly stepped from the shadows made him jump.

'Mr Stokes?' he asked.

'Quick, come inside, out of the light.' He beckoned frantically, then pointed up at the wall. The gallery's security cameras were aimed down at the glistening pavement before them. 'I'm given to understand that it's a matter of national urgency for you to get in here,' he said, shaking his hand. 'Now you listen to me. I don't hold with breaking the law, but it's lucky for you that you're dealing with old friends of mine. I can sympathise with what they've told me, about these toffs holding you to ransom, like. I'm self-taught, me, from Burnley. Came to London in the fifties to make meself a living, and I've done all right. So, as I said, I sympathise. Dr Masters assures me that nothing illegal is to take place, but the fact is that you will be trespassing once we pass beyond this point. I must exact a solemn promise from you, therefore, not to touch any of the exhibits.'

'You have my word on that,' said Vince. Stokes saw how wet and tired the boy was, and a wave of pity swept over him. 'You look done in, lad,' he said, his Lancashire accent broadening in sympathy. 'Bet it's been a pretty rough night for you. Let's get you inside.'

'That's just it,' he replied miserably. 'They haven't called back with the location yet. I don't know where I'm going once I'm in.'

'I don't have a catalogue of all the paintings,' complained Masters. 'How are we supposed to know exactly which picture has lots of crucifixes in it? It's absurd.'

'Can't he just head for the area with the most religious paintings,' asked Stanley Purbrick, kicking Bryant's stray cat out from under his feet, 'triptychs, altar-pieces, that sort of thing?'

'It's a bit hit and miss, isn't it?' said the doctor's wife. 'Maggie, don't you have any bright ideas?'

'Don't encourage her,' complained Purbrick, 'she'll want to hold another séance, and I'm still recovering from the last one. All those talking cats frightened the life out of me.'

'There's something odd about the list,' said Mrs Armitage, her voice muffled as she struggled into a voluminous yellow sweater. 'Surely the bottom name isn't a type of cross, is it? Aimee? Harold, don't you know?'

'I've never heard of one if there is.'

'What about the final line of poetry?' asked Arthur Bryant, removing his bifocals and screwing up his eyes at the sheet of paper Maggie had passed to him. 'You said it was Chaucerian?'

'I think so. I mean, the spelling is mediaeval and the sentiment feels right…'

'Aimee. French for loved?' Bryant tapped a pencil against his teeth. 'Aimee…'

The book Maggie slammed shut made them all jump. That settles it. The others are all definitely crosses. "Aimee" is not a cross.'

'Aimee. A crucifix. Anne Boleyn. What an odd group.'

They made an odd group too, seated in a circle at the dining-room table, hunched beneath twin cones of light thrown by the pair of bronze-green Victorian lamps, books, teacups and wineglasses scattered all about the rumpled tablecloth. Jane Masters rose to her feet. 'I'm going to make some more coffee,' she said with an air of finality. 'Someone should call poor George and let him know that we're stumped. Having got him to agree to this, you can't leave him in the dark.'

It was a bad line, not helped by the fact that Vince answered in a whisper. 'What have you got for me?' he asked, trying to keep their conversation brief.

'Put Mr Stokes on, would you?'

'They want to speak to you.' Vince handed him the phone. Stokes eyed it warily, then gingerly lowered his cheek near it, as though scared of being bitten. 'Hello?'

Masters carefully explained the problem.

'Amy?' asked Stokes, confused. 'Who's Amy?'

'No – Aimee. Wait – the Holbein exhibition, where is it situated?'

'On the lower ground floor at the back. Why?'

'I want you to go there and see if you can see a cross or crosses of some sort. Then call me back.'

'Don't you have any idea where this thing is, then?'

'If I did, I'd tell you, wouldn't I?' said Masters sourly.

The interior of the gallery was as cool and silent as any garden of remembrance. Marble columns tapered into blackness, but it was not possible to switch the lights on without tripping one of the alarms for which Stokes did not have a code. The custodian reached up on tiptoe and opened the steel case of a box behind the reception desk, then punched in a series of numbers that caused a dozen rows of red LEDs to pulse.

'Four minutes,' he explained, 'that's how long we've got before the system overrides my manual command and turns itself back on. It's a fail-safe.' Stokes flicked on his torch and shone the beam ahead. 'Let's go.'

They ran as quietly as they could across sheets of squeaking marble to the rear of the building, then down the staircase to the lower ground level. Merely crossing the floors took over a minute. Vince's boots squealed as he slid to a stop. Stokes raised his torch and a startling image confronted them.

There stood Holbein's portrait of 'Anna Bullen', a tragic English noblewoman dead for over four and a half centuries, immortalised in paint by a German genius. The trusting innocence of her eyes was matched by the simple elegance of her dress. Three thin chains of gold adorned her neck. The hindsight of her tragedy steeped her portrait in an aura of unbearable melancholy.

Stokes shone his light around the walls, illuminating the courtiers and courtesans of a long-forgotten world. The beam skittered across portrait after portrait, briefly subjecting each to scrutiny.

'What about over there?' Stokes suggested.

Vince ran up to the banks of paintings. 'Crosses,' he whispered back. 'Hundreds of the bastards.' There were priests and cardinals, nuns, deacons, bishops, tortured sinners, penitents, martyred saints, crucifixes in almost every painting. 'Damn. How long have we got left?'

'Two and a half minutes. There won't be time to get back -'

'Wait a minute.'

Stokes was puzzled to see him punching out a number on the illuminated buttons of the phone.

'No service.' Vince snapped the mobile shut. 'Payphone?'

'Just over there.' Stokes flicked the torch beam to the far wall. 'You've got to hurry.'