I was born into the watery world of the isles of Scilly - a few paradisiacal crumbs of granite scattered over the turquoise sea thirty miles west of land's end. The islands are dotted with prehistoric tombs. At low water, the remnants of cyclopean walls can be seen snaking from island to island along the bottom of the sea. My mother's family have lived in the islands for six generations.

My childhood was full of tales about a sinking of the land that transformed Scilly from a range of mountains into an archipelago.

In the tales, it was not only Scilly that had sunk. In the mid-18th century, fishermen in the Atlantic trawled up window-frames that they assumed came from a drowned village. A petrified forest lurks deep under the sands of mount's bay, to the south of land's end. These are the remnants of Lyonesse, the British Atlantis, that once lay green and fertile between Cornwall and Scilly.

Just as it was common knowledge on Scilly that Lyonesse had sunk, I knew as a child that Lyonesse had been the home of King Arthur, or Idris as he was called. I was born in a room overlooking the body of water from which the sword Excalibur came, and into which it was flung. The first journey I ever took in this world was from Scilly to the mainland, through what were once the skies of Lyonesse. I grew up with the certainty that Arthur and Tristan and Morgan and the rest of them originated here and in the related Breton Atlantis of Ys in France. I believed in dragons, monsters, and the power of star and stone. While Lyonesse has featured as a springboard for the wilder type of fantasy, it has had no real chronicler. I decided that this was something I needed to put right.

In a properly constituted legendary universe Lyonesse cannot sink as a matter of mere geology. It is human vanity that must cause the inundation, and human virtue that must strive to keep it above water. Idris, I was assured since earliest years, was a good person because he saw that his actions had consequences for others; Murther/Mordred and his monstrous gang were evil because they were interested in the hoggish pursuit of power for its own sake, whatever the consequences for other people.

The battle between the dark and the light lives strongly in my heart. So do the stones and islands of Scilly that are all that remain of the mountains of Lyonesse, and the sun that still shines over Lyonesse, and the gales that still blow there. I visit them often. I am glad that I now have a chance to take you with me.

How did your interest in the tales of Arthur contribute to your writing of this book?
I have read stories from the Arthurian canon in three languages, and in translation from Welsh. One of their many great delights is the way different versions of the same story contradict each other and take on a life of their own. I allowed myself to surmise that they all point back to a common source. And I decided that I would shamelessly invent that common source, using materials that have lain in my head since earliest childhood.

As The Well Between The Worlds is based on Arthurian legend, is there an Arthur figure in your book?
The Arthur (Idris is the earlier form of the name) of Lyonesse is a mythic friend from my childhood: not a Roman chieftain, not a medieval knight-king, but an ordinary boy, roaming a land made with star and stone and threatened by pure evil. On Scilly you can still see the walls of its farms running from island to island. The Mount which you read about in the book still stands in the sea off the Cornish town of Penzance. What my Idris has in common with all other Arthurs, besides parts of his story, is that he will rise to help his land in its hour of greatest need. Apparently that hour has not yet come...

Where did you get your inspiration for the monsters of Lyonesse?
I've always done plenty of fishing. At one moment we lived in a castle in Ireland, and I had the eel fishing rights on the estuary that ran past the base of the crag on which the castle stood. Once we had netted the eels, we stored them in garbage pails. This was fine until someone knocked the pails over. Sitting waist-deep in live eels in a small boat on a big river can provide an unrivalled insight into the lifestyle of the professional monsterfisherman. That, and dealing with a 100lb conger that had somehow coiled itself into one of my lobster traps. Vast wicked bulk, teeth, slime and dreadful speed; this is the true terror of the deep.

Idris starts out as an ordinary boy, but has to follow his destiny away from everything he knows. What drives him to know this is the right things to do?
Most boys know deep inside them that they are the rightful king of somewhere. Most girls know that their closest male friend would indeed be the rightful king of somewhere, if he would only do what they told him. This is the thought that made me start transferring The Well Between the Worlds from my head to paper.

Is there an environmental message in the book?
There is no point in merely telling fairy tales. Good Arthurian stories have always had points to make, and the earliest forms of Arthurian stories are no different. The book was conceived and written on an island while rising sea levels were never far from the news; the story reflects this, I think. The moral in Lyonesse, and perhaps elsewhere, is that if you work with nature, you can keep your head above water. But if you fight nature or try to steal from it, nature will always win. After all, humans are part of nature; it is just that nature has been around for a lot longer than we have. It is fair to describe me as deeply Green, in a waste-not-want-not way. I own a fifth-hand TV, a fifty-year-old guitar, and my newest boat was built twenty years ago. I ride an electric bike, and I would rather sail than anything else, because sailing uses only breeze and tide, and there is plenty of both for everyone.

What's next for Idris Limpet?
The next Monsters of Lyonesse book will be published in Spring 2010. It tells the story of Idris' quest in wicked Aegypt, and how he assembles the first Knights of the Round Table to return to Lyonesse at Darksolstice and do battle with Fisheagle, Murther and the Wellworld...