'There you have me, I'm afraid. Could be five or fifty, although if it's the latter, I imagine there's an inner circle that makes all of the more contentious decisions. An organisation like this tends to have a highly developed internal pecking order providing different levels of information on a need-to-know basis. If you look at the early structure of the Nazi party you'll find pretty much the same thing.' He spotted a cab with its light on and threw out his arm. 'What was the other question?'

'Where do they operate from?"

'I'm sure they recruit at the main universities, and I imagine their membership is swollen by sheer osmosis.'

'What do you mean?'

'Oh, that like-minded individuals naturally drift towards them.'

Funny, Vince thought, that was the same phrase Sebastian used.

'It's pretty much public knowledge that they have a central London meeting-lodge not far from the Holborn Masonic Temple,' the doctor went on. 'I don't have an address for them, though. Didn't get that far, but I daresay I could find it for you easily enough.'

'If you do come across it, could you phone it through to me?'

'Yes, I suppose so. On the condition you don't go doing anything stupid. God, I'd love to see it all brought out into the light. Mind if I ask you a question now? Why would you want to get involved with these people in the first place?'

'I'm a personal friend of the ringleader,' he explained apologetically.

ON A CERTAIN kind of rainy October night some London streets fall back into the past, and it becomes impossible to pinpoint their exact year, like stumbling across an old photograph without a date. On such nights, the dingy dwellings of Spittalfields and Whitechapel still seem to belong to the Huguenot silk-weavers, the prim backstreets of Kensington appear eternally Edwardian, and the houses of the Chelsea embankment, primped with gothic trimmings and standing in Sunday finery like a charabanc of ruddy-faced matrons, remain the province of the Pre-Raphaelites. It is among these latter buildings that the members of the League of Prometheus had set their headquarters.

Although the house itself only dated from 1865, the grounds in which it stood held ancient secrets. Buildings change; sites do not. In later decades London's luminaries attended fashionable parties on its candelabra-filled first floor. Conan Doyle, Whistler, Elgar and Wilde had all taken tea in the Oak Room, a large dark lounge made more depressing by a series of staggeringly ugly morality paintings rendered on fourteen separate oak panels.

Above this room, in the rather inaccurately named Temperance Gallery, Sebastian Wells sat, as was his habit, and watched the setting sun. The air over the river had turned to the clear violet hue that had once been so common above the city before the Industrial Revolution. In those days the seasons were plainly designated, temperatures rising and plunging with clockwork precision. In the winter, the Thames froze over. In summertime, tinder-dry fields caught fire in East London, the billowing pale smoke drifting down to choke the maze of bookshop-filled streets behind St Paul's. But here in this reach of the Thames, at this time of the year, it was impossible to look out and not be reminded of Turner's hazy, iridescent river.

Sebastian stared through the hand-rolled diamond panes, sipping the porter Barwick had fetched him from the kitchens, and thought about Vincent Reynolds.

There could be only one explanation for the young man's sudden change of heart – he had discovered something damaging about the activities of the league. Unease showed in his face, his composure. At their final meeting Vince had avoided pushing for more detailed answers to his questions, as though he was afraid of what he might hear. He had cast his eyes aside, fidgeting like a cat unable to settle, awaiting the payment of the bill.

What had he discovered, though? The league's business activities existed in an area between the land's laws, a mysterious region of favour and reciprocation, of imperceptible nods and understanding smiles, of quietly stopped documents and discreetly passed bills. Isolating a single piece of firm evidence was like sifting through sand. God knows, some very clever journalists had tried and failed.

No, it had to be something personal. Sebastian was aware of several exposed chinks in his armour. The business with the girl was years ago and long forgotten. The pissed old hack who had fallen down the steps then? Or the idiot they had taught a lesson after the concert? The worst part was not knowing.

Sebastian had been careful to show no anger or suspicion that day at the restaurant, but carried out some careful surveillance later. He persuaded St John Warner to check inside the apartment in Tufnell Park. The photocopied notes which lay in his lap made depressing reading.

Perhaps it had been a mistake, trying quite so hard with Vincent. His conversation skills were minimal, his ambitions were low and his background was so common as to be untraceable. Encouragingly, he had a decent reputation among his night-school lecturers for – here he checked St John Warner's notes – 'determination and tenacity'. Not especially popular (any imbecile could be popular, as five minutes spent staring at a television testified) and of course his interests were awash in a groundswell of studenty left-wing ideals, the kind that eventually got crushed out by life's practicalities, but Vince's deep-rooted interest in the history of London manifested itself in the passionate articles he had written. He had earned money in his spare time hosting guided tours through places of historical interest, and had worked for the pleasure of it.

What drew him to study such an unfashionable subject? Why wasn't he out there consuming vast quantities of drugs at raves like every other moron in his social class? Did he manifest some glimmer of originality that rendered him a worthy opponent? The hardest part of any game, of course, was finding an appropriate challenger.

There was an additional personal element to this: Sebastian, the injured party, had been misled, which left him free to nurture a desire for revenge. Any game between them would therefore become a grudge match, for the city they loved and the deceit that had passed between them. Oh, it had the makings of a French prose poem.

Still, there was something tenacious about Vincent that might transform him into a formidable opponent. He was hungry; that was good. The boy had envy in his eyes, and despite his high ideals would probably switch places with him if he could. His biggest flaw would doubtless prove to be his hesitancy; in the moments he wasted deciding what to do, he would forfeit the game. If Sebastian chose to issue the challenge, he would inform his opponent that he intended to play fairly, in strict accordance with the rules. The gentleman would battle the barbarian in the arena of the city they both shared; if nothing else, it would provide an interesting contrast in their playing styles.

He drained the porter-pot and set it down on the sill. The challenge grows, he thought. It is, as yet, unformed. But it is there. It is simply a matter of planning every last detail and waiting for a window of opportunity.

Meanwhile, it had become important to find out what exactly Vincent had uncovered, and if necessary, to stop him from exposing the information.