It's a highways offence I believe. Report it, even if you don't have the reg. could be a local twat with a history.
Reporting something without evidence will likely be a waste of time. The police are hard pushed to do anything even if you turned up with DNA and HD CCTV of someone in the act.
OT but why does the forum auto cencor the F word (fark) but not twat? I use the F word much more than the T word as its much more versatile.