Have you read any other books by Diana Wynne Jones? I used to be quite a fan some years ago, and while nowadays I'm not sure I think they are the best books ever...they're entertaining enough, at least some. Hexwood might be one of my favorites. Or Archer's Goon. (Kyylän Kyydissä in Finnish)
Anyway, I do agree on the movie being...well, I'm not sure I'd go so far as to say it's not as good, but it's not really the same story at all.