Trading standards issue a code of conduct for debt collections companies, which you will find if you google it. The behviour that you decribe breaches a number of the guidelines. I suggest that you write a letter to Orange and to the debt collector, send it registered post, explaingin the situation and enclosing copies of the e-mails.

You should then see if they proceed to take yout o court, where they will undoubtedly lose and you will be abel to sue them.