Mention the people (and even the policeman) who can attest to it being unused.
Ethically if you had used it you would owe it - and you know you did not so you don't.
Be polite but firm on insisting this was not in use and you are not paying. Maybe even a written letter with all the details would be a good idea to send to them. And an insistance that you will not pay for parking you did not use.