Police have declined to prosecute. They say it is simply a matter of opinion as to whether he was driving dangerously close behind me, and the driver claimed in interview that he wasn't beeping aggressively.
Feel utterly let down and very angry.
That route is now basically off limits to me, as they will think they can behave like that with no consequences, and I don't ever want to encounter them again..
On the other hand, the driver has had the inconvenience of explaining himself to the police and is now on their radar so that another complaint would almost certainly have more severe consequences. I'd expect him to be a bit more circumspect around cyclists in future.