You dismissed, while working for you? That worries me. I presume that there was a council involved somewhere along the line and the decisions were made by the council in accordance with the various approved protocols and procedures.
Your former clerk is now a member of the public and subject only to the basic laws of the land. If she wishes to take advantage of the planning enforcement legislation, regardless of her motives, there is nothing you can do about it. I presume you are occupying your caravan in breach of planning law, otherwise the council would not be serving you with an enforcement notice. If I were you, I'd stop worrying about your former clerk and concentrate on clarifying where you are going to be living next year.