As so often, the setting came first. The Great Fire of London raged for four days in September 1666, destroying most of the ancient walled City, including old St Paul’s, the medieval cathedral, and more than 13,000 houses. Seventy thousand people were made homeless, from an estimated population of about 80,000. Forty miles away, in Oxford, the moon turned red and the roar of the flames sounded like the distant sea.

The Fire has become one of London’s defining events – only the Blitz, perhaps, comes near it in the popular imagination. The flames left behind them a smouldering wasteland of ashes, where both criminals and grieving refugees picked their way through the blackened ruins, and where rumours abounded of sinister conspiracies. As modern Londoners know all too well, great cities are vulnerable.

From the start, I knew that St Paul’s would be important, to the novel as to the City. For Londoners, the cathedral has always been more than a building: for centuries, it has been a symbol of the City itself. Its survival during the dark days of the Blitz – encapsulated by Herbert Marsh’s iconic photograph, ‘St Paul’s Survives’ – was a huge boost to wartime morale.

During the Great Fire, the flames surged across the City from Pudding Lane in the east, fanned by a strong wind. The close-packed buildings, the majority built largely of wood, were bone dry after an exceptionally hot, dry summer.

At first it seemed as if St Paul’s were under divine protection. The medieval cathedral was longer and taller than the present building. Many people – particularly the printers and the stationers – stored their valuables there, trusting in the strength of its huge walls.

Ludgate on fire.

Then, in the evening of the third day of the Fire, the flames reached exposed timbers on the roof of the choir. The seasoned oak beams burned at extraordinarily high temperatures. Silver streams of lead ran from the roofs. The great stones sounded like artillery when they cracked in the heat.

Afterwards, it was remarkable how rapidly a form of normality returned.  This was despite the fact that the residential, commercial and manufacturing heart of the Kingdom had been destroyed. What made it even more remarkable was that the country lacked the sort of strong centralised government that could co-ordinate and fund large scale infrastructure projects. Nor was the country politically stable: seventeen years earlier, Londoners had watched the execution of Charles I at Whitehall; it was only six years since the restoration of the monarchy.

Luckily, something of London had survived. The westward march of the Fire had been stopped in Fleet Street. Whitehall, the King’s main residence and therefore the hub of government, was spared, as was Westminster. From Holborn in the north and from the Tower to the east, most of the suburbs were undamaged. The Thames had protected Southwark on the South Bank, which was as large as many provincial cities.

The Ashes of London emerged slowly, with research and writing going hand in hand. The first character I found was James Marwood, a clerk at Whitehall, the largest palace in Europe before its destruction (also by fire) later in the seventeenth century. The second was a young woman, Catherine Lovett, who has a strange fascination with Baroque architecture and a family who consider her as a cross between a social outcast and an asset to be negotiated on the marriage market.

The centre of the novel is a murder mystery: among the long-buried dead of St Paul’s is a very recent corpse, a man with his thumbs knotted behind his back and a knife-wound in his neck. The body is a problem for the Government, terrified that the destruction of the City might be the result of a conspiracy, and that its consequences could soon explode into civil unrest or even rebellion.

The body is also a problem for James Marwood, who has his own secrets to hide, and whose boss is responsible for government propaganda. The dead man is wearing the servant’s livery of Catherine’s uncle, one of the wealthiest merchants in the country.

The Ashes of London was never going to be an easy book to write. Fortunately, there are guides to its vanished world – including Samuel Pepys, of course, whose diaries give an extraordinarily vivid glimpse of London in the 1660s, and of the very texture of the everyday life of the prosperous middle classes. There are plans of lost buildings and detailed surveys of the streets before and after the Fire.

Best of all, there’s London itself – its street pattern largely unchanged, and even its main locations curiously indifferent to flames or bombs or even wholesale redevelopment. There they still are, strung along the Thames just as they were in 1666: Parliament sits at Westminster; the Government rules from Whitehall; and the nation’s wealth is centred on the City.

Andrew Taylor is the author of The Ashes of London.