The Ogre of Oglefort- Eva Ibbotson

When a Hag, an orphan boy and a troll called Ulf get sent to rescue a princess from an ogre, they expect it to be a fairly standard magical mission. But the ogre is depressed and the princess doesn't want to be rescued. The Norns who rule their fates decide to take things in hand - will the Ogre meet a bloody end, or will he get a happy ending?