At first I was dubious considering that I was quite confused and left wondering what on earth was going on. But as the story progressed and the pennies dropped one by one, I found myself thinking that this was quite an enjoyable short story. If only it were longer! If the characters were more developed and the mythosphere well explained, it would be grand. Also, because it was so short a book, the story and even the dialogue felt rushed. But that is, unfortunately, Diana Wynne Jones' way. She has a brilliant premise, such as this, and then neglects to fully expand on the possibilities. Oh well. Such is life.
I did not see the mythology aspect coming when it crept out of the woodwork and into the plot. Well done, DWJ.