…tried in court as if alive…

'They were found guilty of regicide and sentenced to be hung, drawn and quartered. As a mark of public humiliation they were dragged on sledges to Tyburn…'

'The Tyburn route is now Oxford Street, isn't it?'

'Yes, but the actual site of Tyburn tree, the name of the triangular gallows, is at the junction of the Edgware Road and Bayswater Road, although Charles Dickens pointed out that the exact location was still under dispute in his time. There's supposed to be a stone marking the purported spot on the traffic island there.'

'You're both geniuses, thanks,' said Vince.

'Well, not really because -'

'I've no time to spare, Doctor. I'm on my way.'

'You misunderstand me,' said Masters, 'that's not where the clue is sending you at all.'

But Vince had already returned the receiver to its cradle. Even now he was running into the mist enveloping the end of the road.

HAROLD MASTERS stood in the kitchen with his hands in his pockets watching his wife making a cup of tea. Jane had been working at the Victoria and Albert Museum when they'd met. A shy, almost reclusive woman, one of the world's leading experts on Peter Carl Fabergé, she had asked his advice about the supposed rediscovery of a jewelled casket, a legendary 'lost' piece that had vanished during the Russian Revolution, and in doing so had unexpectedly awoken a deep and abiding passion within herself. Finding a strength she had not known she possessed, she had asked him to marry her, and Harold had gratefully accepted. They never did manage to locate the missing Fabergé casket. Jane was his voice of reason, his calm centre. He had already decided that he would take her advice on the matter that was troubling him.

'It's a bit late to start getting people together,' she pointed out, placing the kettle on the stove.

'It's supposed to be for insomniacs, for God's sake!'

'We're not due to meet up until next week, and that's meant to be at Maggie's place, not here. I've hardly got anything for them to eat.' She removed two cups from the cupboard and set them out. 'I suppose you could call around and see who's available. Ring Arthur, he'll definitely be up for it. The poor man never seems to sleep at all.'

Masters slipped his arms around Jane and gave her a quick hug. Anyone else would have considered his idea preposterous. 'I don't just love you because you indulge me, you know,' he said.

'I know.' Jane smiled and began digging about in the refrigerator. 'Go on, then. Get out of my way. Go and make your calls.'

Vince was going to miss his deadline for the fourth challenge, he knew it. Nobody would be safe then. Alighting from the half-empty tube at Marble Arch he made his way up to ground level and exited on the north side of Oxford Street. Any day now the stores would start staying open late for Christmas shoppers, but tonight they were dark and silent. Absurdly postured mannequins bore blank witness as he passed. The great floodlit block of Marble Arch, designed as the main entrance to Buckingham Palace and moved because it was too narrow for coaches to pass through, rose above the traffic, a remnant from a grander time. And there, running through a revolving phalanx of black cabs in the centre of the intersection was – Pam, dressed in a navy-blue two-piece with gold buttons and pink high-heels, looking like a cosmetics representative late for a date.

But he didn't want to see Pam – couldn't see her. There were traffic cameras staring down at every section of the road. She had placed herself in terrible danger coming here. Vince turned away and began hastily walking in the opposite direction, back towards the searing neon lights of the Kentucky Fried Chicken restaurant further along the street.

'Wait, Vince, it's me!' Pam had spotted him. There were few other pedestrians left on the streets. The cameras were bound to pick her up. There was nowhere to hide. He searched the cornices of the buildings; nearly every single one had a small black box at its apex. He imagined the two of them, soft grey figures colliding and talking as their electronic images sprawled across banks of TV monitors. Pam was running full pelt towards him, stilettos tick-tacking across the tarmac, her candy-blonde hair flying about her face. Vince fell back into the unlit doorway of a shoe store, praying she would pass by.

'What are you doing, it's me!' Pam came to a halt in front of him.

'We can't be seen talking,' hissed Vince. 'Do you want to get killed?'

'No one's expecting to see us here, Vince, you're safe.'

'You don't believe me, do you? That I saw somebody murdered tonight? That anyone who talks to me is at risk? They're watching each of the challenge sites.'

'But this isn't one of them,' said Pam breathlessly, 'they're not expecting to see you here. That's what I'm trying to explain, if you'll only have some patience and listen for a minute. You hung up on the doctor too quickly. You're in the wrong place. I came to tell you -'

'Tyburn tree…'

'It's nothing to do with Tyburn! I spoke to Doctor Masters, he was giving you the full story when you jumped to conclusions and cut him short. Hang on.' She pressed a hand against her chest, drawing breath. 'It's recorded that Cromwell and the others were hung up here until sunset, then beheaded. Their remains were supposedly chucked into a pit under the gallows, and their heads were stuck on poles on the roof of Westminster Hall. But Masters reckons it's a trick question. He says that the night before the bodies were taken to Tyburn they were kept at the Red Lion Inn in Holborn, and that a "Tyburn" can mean any place of execution.'

'So they may not have come here?'

'Think about it. Why would they have been dragged east to Holborn when this Tyburn lies to the north-west of Westminster? There were other Tyburns, one where Centre-point now stands, and another in Fetter Lane. The bodies were kept at Red Lion Square and here's the important part – there used to be an obelisk with your Latin inscription on it, standing in a paddock near the square.'

'So the last lines of the riddle refer to what, their ghosts?'

'They've been seen through the centuries, walking diagonally across the square deep in conversation, which is weird because their heads weren't buried there with them.'

…They have no mouths but still must speak…

'Do you have any money on you? I'm nearly out. I'll pay you back if I get out of this alive.'

Pam dug in her shoulder bag. 'I just got paid. Let me come with you, Vince.'

'Absolutely not. I won't be responsible for something bad happening to you. You can't risk being seen with me.'

Pam stopped and stared at him awkwardly. 'You are sure about all of this, aren't you?'

'What do you mean?'

'I mean you're sure you really saw someone – killed?'

'Well for God's sake you don't think I'm making it all up, do you?'

'Well no, it's just that -'

Vince tore the riddle-page from his jacket and waved it in front of Pam's alarmed eyes. 'You think I sat at home writing these damned things out, do you?' He became aware that he was holding a scrap of paper no bigger than a postage stamp. The rest had fallen to pieces in his pocket. Pam had edged out into the street and was now standing within range of the traffic cameras. 'Get back in here!'

'No, let's test out your theory.' She looked about at the passing traffic, then walked towards the nearest intersection. 'Let's see if we can make this secret society show itself.'

She really didn't believe in any of it, not deep down. His best friend didn't believe him. Perhaps it was better this way. Thank God she hadn't brought Louie with her; they would really have stood out in a crowd.