I've described this incident before on the forums but let me re-iterate.
About five years ago a passenger in an overtaking car punched me on the ear. I didn't fall off and was not injured, but I got the numberplate and so I called the police.
The driver was traced and apparently shopped his own passenger. This is what I was told by the police. As a consequence, I gave the police a statement and the passenger was brought to book and cautioned, but the driver got off scot-free. Maybe this was right - maybe not. I was satisfied with the outcome.