What can I do to add value to the park if caravans on site do not have planning permission?

You could consider making an application for a certificate of lawful use to the local planning authority if it is too late for the local planning authority to …take action to enforce the breach of planning control. Factual evidence would need to be provided to the local planning authority in the form of a statutory declaration about the stationing of the caravans. Be aware that if your application were to fail, the Council could take enforcement action in respect of the breach of planning control.