Penalty for false parking – but I’m not on site at all

Hello everyone, yesterday I received a letter dated 9.12.2010, in which I was accused of false parking on 2.12.2010 in the time of 7:10-7:20. I was parked on a disabled parking lot in an area where I was not actually staying, which was also designated as such – allegedly. Since the accusation is about a Thursday=workday, I was on the highway towards the workplace about this time. That I filed an objection is clear. But do I have to pay attention to something? What does the evidence look like in such a case? Thank you for your answers.