"... if he'd threatened you with a non-motorised weapon, they would take greater interest, or perhaps not"
Equivalent situation, man swings baseball bat at another man who ducks out of the way and isn't hit and there are no witnesses to it.
As Mr Bodna says, sadly there really isn't anything the police can do. With no witnesses, and no injuries, it genuinely is one man's word against another. The police are in a pretty much impossible position with this - think of the alternative, they report to the fiscal for investigation every single claim of wrongdoing that has no evidence to support it.
It does show again though why so many people (myself included) ride with a camera attached (not that it would show the homicidal intent necessarily, but it would show the collision and the driving off - depending on the mount/angle it might get the 'trying to drive off with someone underneath the van' as well, which would then go some way to showing the intent).