sort of similar.... the towtruck could only leave my car in a no standing zone outside the mechanics at night, as it was closed. went down the next day and left the keys with them but the lazy pricks didnt move the car for 2 days.
Anyway I coped a fine, and I didn't even have the keys to the car. I've sent a letter enclosing the copy of the towtruck receipt and the invoice for the repair to the infringement bureau, but apparently this isn't sufficent evidence (what the f**k is, I ask....) anyway I'm calling them in tomorrow to find out what other possible evidence they need that A. the car could not be moved and B. I didn't have the bloody keys....
write a letter and enclose as much evidence as you can... you might just get off...