The library didn't have that much by Wynne Jones that wasn't part of a series, so this was all I got. I enjoyed it, and it could, potentially, make a very interesting and slightly dark film, a bit Gaimanesque.... but it could also be completely ruined by a filmmaker, so I dunno... I'm thinking that a film could/would/should edit out some of the longer passages that get a little slow. (However, mustn't be too critical, she writes for kids).
A world of magic is held hostage by a businessman from another world (ours?) who forces them to arrange fantasy tours every year, something that disrupts everything. They're finding it harder and harder to bounce back from the havoc these tours wreck. By selecting the "wrong" people for key posts they hope to sabotage it enough to make the businessman uninterested and go away. Instead the effects become more far-reaching. Good, funny, sometimes disturbing. It'd be nice if she developed the darker bits more.