I had a quick look round the web for clarification, and it appears that actually, I am writing a series, not a sequel. I found some pretty useful information, like this - how to write a great series - from NY Book Editors. And this - mistakes to avoid - from Now Novel.
I suppose I wrote the first book of the series - working title The Mage of Merjan - so it could be read as a stand alone title, because I had no idea when I began to write it whether I or it was ever going to be published. (Fun fact: by comparison, StarMark started of as a series of three books but came together as one complete story.)
But in my head, the original story of this particular series was always much bigger than one book. There were five regions on the island where the story's set, and I wanted my main character to visit each of them in turn. Each visit to a new region would present a fresh adventure for the main character while continuing to expose them to a formidable enemy.
Incidentally, I was told by an agent in a 1-2-1 once that there was no point in writing this series as five books. I should just have three, because he said so. I smiled sweetly, bit my tongue (Harry Potter has seven, books, so what's wrong with longer series?) and carried on anyway.
When I read the articles on series writing, I was relieved to see that my approach - to plan the whole series pretty much up front - was actually a Good Thing to do, especially for fantasy. It means there's one BIG story arc which overlays however many books, but underneath are multiple smaller story arcs, with each book finishing off at least one. This is different to the kind of series where each book presents a complete and separate incident for the same central character - something like Agatha Christie's Poirot, for example.
So where am I up to with book number two of five? The outline is planned - I know what's going to happen and to who, and where the story finishes ready to move on into book three - and I've begun to type it up.
Already, one difficulty is deciding how much book one story to repeat in book two, so that readers new to the series are informed, but those who've read book one won't be going over too much old ground. It's a fine balance, and will probably have to be tested at the beta reading stage by those who have read The Mage of Merjan and by some who haven't.
I've also noticed a new addition to the Doubt Demon family since I've begun the typing up. It's only a little fella, but he keeps whispering 'what's the point of writing the second book if you don't know whether the first will ever be published?'
? ? ? ? ? ?
I can't give him the answer, so I'll ignore him as much as I can at the moment. And in the meantime, I'll carry on writing the continuation of Tilda's story in the hope that someone, somewhere, will like the concept enough to publish the entire series.