The wolves of Willoughby Chase by Joan Aitken

Set a long time ago (at a time that never happened) England was overrun by wolves, but as Bonnie and her cousin discover there was more danger at home than the wolves, as their new Governess Miss Slightcap seemed most unpleasant- she would shut Bonnie in a cupboard and she fired all the friendly old servants and sent the cousins away. Can Bonnie and her cousin outwit her and her network of criminal freinds?