I mention this conversation – which began on a cold evening in late September 1890 – not because of the wisdom of my remarks, as there was little in them, nor because they accurately reflected my views, as they did not. Rather, it was because of the intervention by Abraham Netscher, then head of the Banque de Paris et des Pays Bas and an infrequent visitor to Elizabeth's house.

'I think my two friends here are shadow boxing,' he said comfortably, sipping from his glass of brandy. He was a fine man, tall and impressive in his stance, with a high-domed forehead and a piercing stare. In fact, this was because he was short-sighted and was too vain to wear his glasses in company. Accordingly he had a tendency to peer at people, and sometimes to stare a little above the right or left shoulder of the person he was addressing, a habit which was remarkably disconcerting as it gave the impression always that he was talking to someone else.

In many people such vanity would be undignified, but no one would ever have thought this of M. Netscher, once they knew him a little. He was a man of exceptional intelligence, and allied this capacity to great wisdom and immense experience, having lived through – and prospered through – several regimes and generations of politicians.

'You are both defining the notion of war far too narrowly,' he remarked, 'and in a fashion which, if you will forgive me for saying so, is remarkably old-fashioned for people who are so young.' Netscher was somewhere near his seventieth year at this point, but still had a good decade left before his failing powers obliged him to rest.

'The marching of soldiers is usually to steal territory or money from the opponent. But in this case, as you say, neither France nor England have any such designs on the other. This is not because nations are any less greedy, I fear, but because wealth no longer lies in land or treasure. France could, perhaps, invade and annexe all of Cornwall, or Scotland and Ireland, and it would scarcely damage England in any way, except in its pride. Its power lies in its accumulated wealth, and that cannot be stolen by armies. London is the centre of the world of money. It is an empire on its own; in fact the real Empire only exists to serve the needs of London. From the incessant movement of capital comes all of England's power.

'But it is fragile, this strength. Never in the whole of humanity has so much power been generated by such a feeble instrument. The flow of capital and the generation of profit depend on confidence. The belief that the word of a London banker is his bond. On that evanescent assurance depends all industry, all trade and the very Empire itself. A determined and sensible enemy would not waste his time and gold striking at the navy or invading the colonies. He would aim to destroy the reputation of a handful of bankers in London. Then the power of England would dissolve like mist on a warm morning.'

'I think such an enemy would discover that it is more resilient than you say, sir,' I suggested. 'Just because it is a power that cannot be touched or held, does not mean that it is not real or strong. The most enduring institution in the world is the Church, which depends on faith alone to survive, and it has survived empire after empire for nearly two thousand years. I would be quite content if the influence of the City of London lasted half as long.'

'That is true,' he conceded. 'Though with the Pope locked in the Vatican by Italian troops, and priests being expelled from schools across Europe, and the teaching of the Church being challenged by historians and linguists and scientists across the world, I do not find that you greatly strengthen your argument. I doubt I will live long enough to see the last church close its doors forever; but you may.'

The old man piqued my interest. I wondered for a moment whether he had even come that evening specifically to have that conversation with me, but eventually dismissed the notion. My secret life was unassailable, I was sure. No one connected me – the associate of aristocrats and princes, the dilettante journalist for The Times – with the occupant of the little office in the rue Rameau who bought and sold information from diplomats, soldiers and other spies. Nonetheless, I dwelt on his words for several days, and the more I considered them, the more I believed that his words had reflected something he had heard, or half heard.

Could such an attempt succeed? Not in the way that M. Netscher said, of course; he was exaggerating there. But it was certainly true that inflicting severe damage on the City of London would be more harmful than defeating England's army, were it ever so foolish as to join battle with any other than half-armed natives. Every week, hundreds of millions of pounds flowed through London, its banks and discount houses, clearing houses and depositories. The whole world raised its loans through the City. The decision of a banker could determine the outcome of a war, or whether that war would take place. Wars were fought on credit; cut off the credit and the army must stop dead in its tracks as surely as if it had run out of food or ammunition.

Attacking the reputation of the City could be relatively inexpensive and have no consequences if it failed. But how could it be done? I could not see it. 'If it can be imagined, it can be achieved.' Was anyone doing the imagining? I thought about it for several days, then realised that mere thinking would accomplish nothing. I had to do some work.

Discovering anything by examining the career of M. Netscher was fruitless, it turned out: he had been around for such a long time that he knew absolutely everyone, and heard everything. There was no simple solution there; so I had to go back over the last few years and discover who his enemies and rivals were; this also produced nothing of any great interest. Such musings came over the following days, however; and that particular evening ended without anything else of interest. I did, however, write a short report on the conversation and send it to Wilkinson – I was a good bureaucrat already, and realised the importance passing on responsibility for things I could do nothing about.

Thursday evening became part of my life, something I looked forward to and enjoyed, partly for the conversation, but more for Elizabeth, whose presence I came to find oddly comforting. I took pleasure in watching her in what was now her natural habitat, so to speak, the way she could conduct a gathering like a maestro, discreetly and without ever imposing herself. I watched with something close to affection as she relaxed ever more into her role, became more sure of herself, more adept at her profession. In general I quite forgot what exactly that profession was. It was impossible to think of her as anything other than that which she wished to be.

One evening, though, the salon ended differently. She had been quiet, unusually reserved all evening; her admirers appeared to feel they had been given short weight. Ordinarily, she would have risen to the challenge, drawn them out, calmed them down, flattered and reassured; this evening she seemed strained and almost ill at ease, almost as though she wished they would go away.

And eventually they all did, except for me; she signalled quietly that she wished me to remain, so I held back until we were alone, the door shut to the world outside. I wondered for a moment whether the evening was going to turn into a night of excitement, but it rapidly became clear that she had – for her – a greater intimacy in mind.

'I am afraid I feel ashamed of myself,' she said, once we moved into the little salon, which she kept for herself alone. 'When I said I would not help you in your work, I did not dream that I might need help. And now I do.'

'I, in contrast, am delighted. How can I be of service?'

'My diaries have disappeared. And so has Simon.'