In 2009, Ms Wiz reached her 21st birthday - 21 years of having stories written about her, that is. I found myself thinking about how she has been my imaginary friend for over two decades, during which many other things in my life changed.
I wrote about the experience in the Autumn 2009 edition of The Author.
There is something not quite right about those who write for children. Most of them, surely we can agree, have a small but significant psychological flaw which draws them back to childhood. As a result, creating fiction for young readers is more instinctive, more personality-based, than writing for adults and, in my personal belief, cannot be forced or learned if the spark is not there. You have to be slightly odd.
I have been thinking a lot recently about the joys and perils of writing for children because an old friend, a colleague of mine in this area, has just reached her 21st birthday. Not that Ms Wiz, the character I invented in 1988, actually does birthdays; she has magical powers and tends to be sensitive about age and ageing, as anyone who happens to be several thousand years old has a right to be.
She had started life as Miss Heccatty, a character in a picture book I had planned. The story received a universal thumbs-down, but Brenda Gardner of Piccadilly Press suggested that I might try writing for slightly older readers. On the basis of a one-page outline (would this happen now?), she commissioned three stories about a character now called Ms Wiz. They would be around 6000 words each.
Ms Wiz has changed over the years, becoming vainer, dottier and generally more capricious, but then so has the world outside. The nine-year-old girl for whom the early stories were largely written, my daughter Alice, is now herself the mother of a two-year-old boy.
During that time - this was a shock - writing for children has actually become fashionable. Notions of what is acceptable and appropriate in young fiction have shifted. Two decades ago, my character was politically correct well before that concept took hold. She refused to be described as a witch, preferring the more neutral term 'paranormal operative' and once when impersonating Queen Elizabeth I (she had returned to the 16th century), she found it physically impossible to make the famous speech which opens, 'I may have the body of a weak and feeble woman.'
All the same, there were elements in those early stories which would raise eyebrows today. One of the classroom heroes was a fat boy called Podge. Librarians were shy, policemen were stupid, parents were insanely ambitious (the series has cheerfully embraced clichés).
Podge and the other children have remained the same age over the past two decades but their personalities, crazes and interests have changed to reflect our bright new world of celebrities, reality and talent shows on TV, mobiles and computers. I am more careful with my jokes, more aware that there should be a racial mix in the stories.
Ms Wiz was never meant to last as long as she has, and her career has been marked by several short-lived retirements. After six books, she was waving goodbye to readers as she sat under a palm tree on the tropical island of Sombrero. Three stories later, in Ms Wiz Loves Dracula, she was faced with a tough choice between magic and marriage (and, yes, that joke was made). To the disapproval of many of her readers, she chose to get married.
It was a terrible mistake. When, two years later, she returned, it was with a baby who also had magical powers. I discovered that, in fiction as in life, babies can be a serious impediment to action and adventure. The husband, a school inspector called Mr Arnold, turned out to be a flop, too. Good-looking but dull, he disapproved of his wife's magical powers. I seriously considered writing Ms Wiz Gets Divorced but was dissuaded by my concerned editor, Marion Lloyd at Macmillan.
Writing stories the series has taught me some other useful lessons. I have been reminded that the best writing has a strong element of play to it, that stories should keep moving forward, that hobbyhorses must be avoided at all costs.
Another less cheerful realisation has been that, however familiar with the material and format I have believed myself to be, I am never entirely in control. Whenever I have begun to think that at least one type of writing is easy, I hit a wall. As in the stories themselves, Ms Wiz's magic is sometimes nowhere to be found.
I remember hearing Penelope Lively say in an interview that it was not that she had given up writing children's fiction, but that it had given up her. Storytelling, I have been uneasily aware, is a mysterious gift which can disappear overnight.
The series has been lucky. It has had two brilliant illustrators, Kate Simpson and Tony Ross. There has been an unusual level of publishing continuity, from Piccadilly to Macmillan to Andersen Press. Foreign editions have been published, and there was once even some quite serious talk about a musical feature film. That ended predictably enough: one moment, it was just a question of which Jennifer, Aniston or Lopez, would be playing Ms Wiz, the next - silence.
The real and consistent joy in writing about Ms Wiz has been her readers. It is a peculiar privilege to write stories for children who are starting to read books by themselves and their reactions, in classrooms and in letters, has been generous and cheerfully - sometimes brutally - honest.
I remember being surprised when the late Humphrey Carpenter, who had written so many highly praised books for adults, said in an interview that his Mr Majeika stories were probably the work of which he was most proud. Now I understand a little better. Writing these books may not use as many literary muscles as writing for older readers but they contain the essence of story-telling, and offer the very best kind of relationship between writer and reader.
There are more of my columns for The Author here.