Why the name?  -- of course, there’s a natural association between the word ‘Commonwealth’, and the English Civil War, the time period in which the novel is set, which ended in precisely that. But I actually chose my title because of a pre-existing book, The Secret Commonwealth of Elves, Fauns and Fairies. This was written by a Scottish minister called Robert Kirk, in 1691. He was a Gaelic scholar and folklorist, and his Secret Commonwealth is a wide-ranging and broad-minded treatment of Scottish supernatural lore, both Gaelic and Lowland Scots. He corresponded with the scientist Robert Boyle on the subject of second sight – Boyle was interested in this – and he was clearly anything but bigoted.

A legend grew up after Kirk’s death that he had been spirited away by the fairies for revealing their mysteries. In Walter Scott’s version, ‘after the ceremony of a seeming funeral, the form of the Rev. Robert Kirk appeared to a relation, and commanded him to go to Grahame of Duchray. “Say to Duchray, who is my cousin as well as your own, that I am not dead, but a captive in Fairyland; and only one chance remains for my liberation. When the posthumous child, of which my wife has been delivered since my disappearance, shall be brought to baptism, I will appear in the room, when, if Duchray shall throw over my head the knife or dirk which he holds in his hand, I may be restored to society; but if this is neglected, I am lost for ever.” True to his tryst, Mr. Kirk did appear at the christening and “was visibly seen”; but Duchray was so astonished that he did not throw his dirk over the head of the appearance.’ Thus the unfortunate minister disappeared from the human world forever.

I found him very useful, though I didn’t follow him in every respect, and his prose is often charming. He says, ‘These Siths or Fairies they call Sleagh Maith or the Good People […] are said to be of middle nature between Man and Angel, as were Daemons thought to be of old; of intelligent fluidous Spirits, and light changeable bodies (lyke those called Astral) somewhat of the nature of a condensed cloud, and best seen in twilight. These bodies be so pliable through the sublety of Spirits that agitate them, that they can make them appear or disappear at pleasure.’ I took the idea that they could change, but not that they are insubstantial – other sources, such as the stories of Tam Lin and Thomas the Rhymer, in which the queen of fairy takes a mortal lover, suggest that they seemed pretty solid. One hint which I explored was that their morality might have a completely different basis from that of seventeenth-century Christians: ‘They live much longer than wee; yet die at last, or [at] least vanish from that State. 'Tis ane of their Tenets, that nothing perisheth, but (as the Sun and Year) every Thing goes in a Circle, lesser or greater, and is renewed and refreshed in its Revolutions …  they are said to have aristocraticall Rulers and Laws, but no discernible Religion, Love, or Devotion towards God’.