i would imagine that if you kept it factual and kept calm, although showed some concern then the Police would take this seriously?
im sure they would do the same if they had a report of a motorcyclist riding in the same manner.
hopefully you can have a word, and this may save some poor sole from having a head on with him!.....
i would have no problems reporting this if this was me.

hope you get it sorted, and thanks for looking out for all of us :-)