Puternut, it is somewhat confusing how the situation has been described (sorry headteacher), and the issues regarding the emailing, in particular the reason for sending the emails, even soon after an official police caution for harrassment.
Headteacher, I am afraid you do not come across as being innocent to the allegation of crim dam to your ex-partners vehicle and you certainly have the motive. On the face of it, you are engaging in a dangerous game, one that you will not win. The police officer has disclosed that your ex has evidence of the offence which they are prepared to run with and I can not help thinking that the police will run circles around you in any formal interviews. As stated several times, you need legal advice.
I wont go on but this situation is aggravated by it being a domestic violence related incident and you are in whats known as a 'notifiable occupation'. (search google etc).