AS VINCE walked wearily into High Holborn, sooty snow-flakes began slanting through the sweeping grey skies like television static. His breath drifted before his face in clouds. He brought out the latest letter and examined the envelope. Tearing open the dampened vellum, he found, as usual, a single square of chemically treated white paper. Three lines of type adorned the sheet:

The Challenge Of The Crenellated Pachyderm

Nine Trees In The Nineteenth Reach

Opposite The Secret Five

Pray Remember The 25th Of July

To Three Of Four Doors

And Up To Steel And Stars

Time allowed: 120 Minutes

More gibberish that he was expected to unravel into something tangible. Not much to go on here, but it would have to do. Two hours allowed again. It was almost as if they were regulating the timing of his movements to some specific undisclosed agenda. He checked the batteries on the mobile phone; they were standing up at the moment, but he would have to choose his calls wisely. His boots were starting to let in water. The slush-puddled street in which he stood was empty in either direction. He hoped Pam had gone home and was safely tucked beneath her duvet. It was best to face the rest of this night alone.

…Nine Trees In The Nineteenth Reach…

The Thames was divided into reaches, but he had no idea how many there were. Who would know? His Uncle Mack might. His father's younger brothers had both worked on the river until one of them had fallen from the stern of a tug and drowned. Mack had filled his head with lurid stories as a child, always showing him the kindness his father had never managed. Vince opened the phone, thought for a moment and punched out a number.

Dinner had been disappointing. Barwick really was the most frightful cook. Anything more complex than a steak was beyond him. Thank God the cellar was still well-stocked. Sebastian reached for another bottle of Montrachet while Ross Caton-James, seated at the far end of the table, adjusted his portable TV screen.

'He's on the bloody phone again,' he exclaimed, tapping the blurry monochrome image on the monitor. 'It really is too much. You're going to have to take it away from him.'

'No doubt he prides himself on being a part of the modern world. We shouldn't have made the questions so academic, Bunter, then he wouldn't need to consult people all the time.'

'The next one is hardly academic.'

Sebastian snorted in disgust. 'It's quite the least appropriate of them all, but it was the best that poor old Barwick could come up with. The fellow's hardly a mental giant, but he demanded to have his turn.'

'Oh, I don't know,' said Caton-James airily, 'I think it has a certain panache. I'm surprised he figured it out by himself.'

'But will Reynolds, though?'

Sebastian gestured vaguely into the air. 'He's working class, he's supposed to know about that sort of thing.'

'He's taking longer than you thought, isn't he?' said Caton-James, needling.

Sebastian was agitated. In truth he had expected a faster response from Vince, a little more ingenuity. He was merely plodding from one problem to the next, whining to his friends until they solved each challenge for him. He was supposed to be running in terror for his life. You'd have thought that seeing someone killed before his eyes might gee him up a bit. He needed more of a spark from his key player. It was inconceivable to think of failure now.

'There's no sport in this, Bunter. We have to do something. I don't think the demise of Mr Street-Trash was enough to shake him.'

'If this one proves too easy to crack, we can change the running order.'

'Good show.'

'One thing bothers me,' said Caton-James, fiddling with the TV controls. 'The weather's having an adverse effect on our signal. It's hard to make out who's in some of these shots, and quite a few of the cameras are getting snow on their lenses. There's nothing we can do about that.'

Sebastian rose and wandered over to the mullioned window where Barwick sat, miserably staring down into the deserted streets. 'Snow in London before Christmas – a rare thing. No wonder it's so sodding cold. Stir yourself, Barwick, you gormless protozoid, stoke up the fire, get me a drink, make toast or something. Don't just sit there like the sad corpulent lump you are. We still have a long night ahead of us.'

'Gravesend, Northfleet Hope, St Clements, Long, Erith Rands…' Mack Reynolds read from the maritime manual he had not opened in years, 'you count the reaches of the Thames from the sea inwards.'

'I need to know what the nineteenth is called.'

'Hold your horses, young man. I don't hear from you for nearly three months, then when you do call it's after midnight, and you can only stay on for a minute.'

'I told you, this is an emergency situation. I'm taking a terrible risk just talking to you.'

'You're not taking drugs, are you? You know, after your father died I promised your mother to try and…'

Vince sighed. 'Mack, I don't have time for this right now, mate. I promise I'll visit you.' If I'm still alive in the morning, he added under his breath.

'All right, let's see. Give me a minute. Halfway. Barking. Gallions. Must be further along.' He heard Mack rustling the pages, trying to hurry for his sake. 'Ah, here. Limehouse, Lower Pool, Upper Pool, London Bridge. No, can't do it.'

'What do you mean, you can't do it?'

'The nineteenth has no official name.'

'Are you sure?'

'Absolutely. From London Bridge to Westminster and from Westminster to Vauxhall Bridge are the only two reaches that are unnamed.'

'What if you don't count the unnamed ones?'

'Hang on.' The line started crackling. He prayed it would hold. 'That would make the nineteenth – Nine Elms.'

Nine Elms, near Lambeth Palace, where the Covent Garden market had relocated. He should have known. It was clear to him now. 'Thanks, Mack, you're a wonder.'

'Call me soon. Keep your promise, you hear?' His voice was cutting out.

'I will.'

'You've said that before, Vincent.'

'I mean it this time. I have to go.'

'You always have to go.' The old man chuckled, then the line failed. It was probably a bad reception area. The mobile's battery had enough life in it for a few more calls. He would try to rely on call boxes wherever possible. Vince checked his watch. 12:23 a.m. It would take him at least half an hour to get to Nine Elms. His deadline was up at 2:00 a.m. The snow was trying to settle. Turning up the collar of his soaked jacket, he set off in the direction of the tube station once more, hoping to catch the last train south.

Cities bisected by rivers often develop distinct characteristics in their separated halves, and London was no exception. South was low ground, traditionally poorer, now patchily gentrified, but with a unique nature of its own. Here a car was called a 'motor', problems were 'sorted', people would 'see you all right'. Vince knew that a rough guide to an area's wealth was provided by McDonald's outlets; the larger the yellow arches, the poorer the neighbourhood. Hampstead would only allow a discreet McDonald's symbol outside its much fought-against outlet. South London had giant drive-through branches.

On the southern side of Vauxhall Bridge stood the next part of the clue – Opposite The Secret Five. This was obviously a reference to the Secret Five of Nine Elms, the headquarters of the British Secret Service, MI5, housed in an extraordinary cream-coloured wedding cake of a building that looked more like a latter-day redesign of St Clement Dane's, with Cyprus trees added to the Thameside upper levels, rather than a government office. The architecture of the edifice was absurdly high-profile for an organisation dedicated to the creation of covert operations. Everyone knew what it housed. It was a very public secret.