The Secret of Platform 13- Eva Ibbotson

Under Platform 13 at Kings Cross Station there is a secret door… It appears only once every nine years. When it opens, four mysterious figures step into the streets of London. A wizard, an ogre, a fey and a young hag have come to find the prince of their kingdom, stolen as a baby nine years before. But the prince has become a horrible rich boy called Raymond Trottle who is determined not to be rescued.