Searching for this book for a long time.
A fantasy book I read ages 9-11, so in my school library around 2000-2003.
It was a dark sort of fantasy, for a children's book. A key component was a magical vale only accessible by a tunnel (a train tunnel I think). I remember vividly a passage about a jet plane flying over the valley, but the people within dismissing it as not important to them.
I remember as well something about a labyrinth described as being like the shell of a snail, with a tiger-like monster within it. Though it's possible I conflated this image from another book.
I have ruled out The Spiral Maze by Patricia Bow. I think I have ruled out The Magic Apostrophe, by Jenny Sullivan, but I know that series does feature a magic land, so it could be a later book?