As the first reviewer has already given an exhaustive precis of this work I will just add that it is a delightful Christmas thriller and the BBC TV version is a wonderful and faithful retelling of the story.
A forgotten classic - among the 10 best children's thrillers, IMHO
Set in 1930's England, it follows the Christmas holiday adventures of Kay Hawker and his cousins Peter, Susan, and Jemima. On the train home from school, Kay meets a stange and kindly old man. As they become friends, the old man gives Kay a precious treasure to guard, the Box of Delights. It is a magic instrument that can shrink you to the size of an insect, can make you fly, and can take you backward or forward through time. It turns out the old man has an unusual past, and there are ambitious villians after him and the box. Because the villians are disguised and passing themselves of as men of the cloth, the children have a difficult time exposing them. They are resourseful, however, and succeed in their quest to save the box and their Christmas celebrations. This story incorporates the mythical heroism of legend ( ie King Arthur, the Trojan War, Hern the Hunter and Fairy folk) with contemporary bad guys, car chases, kidnappings, and rescues. An excellant British/Canadian television special was produced about fifteen years ago that brings this story to life. Either are very enjoyable for boys and girls, and people of all ages.
but beware: the editions present on Amazon are an heavily abridged version edited by Patricia Crampton (it's an english tv-movie tie-in, if i'm not wrong).
Check Amazon.co.uk for the complete version (published by Egmont)of these two wonderful, ageless