Phone call to the council. stating that you have an independent witness, that saw the damage take place, and also believe that the Driver was unaware that damage had taken place, this stops the driver getting into trouble... make sure any paint transfer is still evident, be prepared take a trip with the car to the compound where the truck is kept
In my experience, Councils are very forgiving about damage caused, and are happy to get the problem resolved asap
Ben, its also likely that the Driver has reported the incident, and are waiting for someone to come forward!
Thom