We all know it was an illegal act, as the ref to the 'unmarked police car' above confirms. You have their number. Why not report it to the police? I know you will feel it probably a waste of time as they would be unlikely to commit manpower to following up such an incident; but at least you would feel you had done something to try to obviate such illegal and anti-social behaviour, which is supposed to be every citizen's duty.

~M~