@PASmith: cellphone or small digicam. photograph license plates, scatter anonymously/send to police/upload to neighbourhood watch website/whatever.
here in norway when something like that happens people start breaking out cameras and recording it all, from what i've seen (last month we had a guy start beating out a woman outside our apartment and he got filmed by multiple bystanders and neighbours as well as there being attempts to stop him and chase him when he escaped. i very disgracefully did not join in, which i now regret, and will not make the same mistake in the future).
here in norway when something like that happens people start breaking out cameras and recording it all, from what i've seen (last month we had a guy start beating out a woman outside our apartment and he got filmed by multiple bystanders and neighbours as well as there being attempts to stop him and chase him when he escaped. i very disgracefully did not join in, which i now regret, and will not make the same mistake in the future).