It is January and I am arriving at a country house in Yorkshire. Fog and rain shroud the park. The interior is a dim labyrinth of splendid but desolate rooms, full of winter shadows and echoing footsteps. It is everything that pleases me best: the perfect setting for a pseudo 19th-century novel.

The phone conversations about a possible TV series of Jonathan Strange & Mr Norrell stretch back years, but now that the moment has come, now that I am actually here at Wentworth Woodhouse, I lose my bearings. It is not just the scale of the thing that is unnerving. (Look at that positive battalion of lorries drawn up in front of the house – perhaps I should find someone and apologise for all the trouble?) Nor is it simply the sense of otherworldliness, of reality out of joint with itself – I imagine all film sets have something of that quality. (Look at the miles of electrical cable that wind up and down stone staircases and disappear, serpent-like, into the darkness of one of a hundred nameless rooms.) No, what I find most bewildering are the people with early-19th-century hairstyles and early‑19th-century clothes. I suppose I ought to have expected them, and it’s not that there are so many really, not in comparison with the film crew.

But nothing, I find, has prepared me for the sight of my own characters walking about. A playwright or screenwriter must expect it; a novelist doesn’t and naturally concludes that she has gone mad. (What do they need so many umbrellas for? Don’t they realise that they are imaginary?)

Jonathan Strange & Mr Norrell BBC1 launch trailer

In the part of Wentworth Woodhouse that has been made to look like the House of Commons, Sir Walter Pole smiles and saunters over to speak to me. In a ballroom of immense magnificence Lady Pole and Mrs Strange perform a dance of their own invention; it is both graceful and funny. (Later someone will give me a photograph of it.) Stephen Black looks grave and self-possessed and keeps to the shadows. Childermass – in straightforward Yorkshire fashion – shows me his tarot cards and lets me hold them for a moment: they feel warm and pleasantly rough in the hand. Out of the assembled ranks of fairy dancers the gentleman with the thistle-down hair gives me a friendly wave. (This last, I am willing to admit, is not the least in character.)

The two people I do not meet – slightly to my relief – are Jonathan Strange and Gilbert Norrell. I know that I do not meet them because the defining characteristics of Strange and Norrell are arrogance and self-regard. (I am sorry to have to say this about people I am related to, but it is true.) The people I meet in their place – Bertie Carvel and Eddie Marsan – are nothing of the sort, being warm and delightful.

A month or so later and we are gathered in York outside the Minster. People are walking home from work, driving their cars through the city centre, eating in restaurants. Everything is quite as normal. Except that in just the one spot – before the great west front – a snowstorm is blowing; and battling through it is a covey of black-coated magicians in three-cornered hats with lanterns in their hands. Mr Norrell is about to do magic in York Minster again. People stop and stare. In the middle of 21st-century York on an ordinary weekday evening there is suddenly a strange little bubble of 19th-century-England-that-never-was.

The next day I am walking with a friend outside the Minster. I look down and see some faint white traces in the cracks between the paving stones.

“Oh, look,” I say. “This is my snow.’’