Thanks for the reply kev, and don't worry I didn't go home and ring 999! There are ways of reporting drivers for dangerous behaviour other than 999 as this clearly wasn't an emergency.
And of course I would of like to have explained to this chap the stupid manor of his driving when we were at the round about. A few choice words hopefully would have done the trick, however this guy looked rather worried that I caught up with him on traffic after such a stunt, and just sped off as soon as he could. Obviously I wasn't going to follow him, it's not worth it. But my opportunity to express myself had gone and passed as he continued to drive like a idiot into the night. Hence the name and shame on here.
And yes there are far worse things that happen on the road, but this shouldn't be excused either.
Thanks for your response