Oh I’m so stupid. Yes, I should have said in the previous post that I’m rendering with mental ray out of Maya, and I was using the 2D Motion Vectors render pass. What I should have been using was the Normalized 2D Motion Vector pass, I believe. This looks more like what you’re after, right?:

And of course it has solved all my issues:

Sorry to have wasted time on such a simple issue, the tutorial linked on your site clearly says to use the normalized pass, I just hurried over that apparently. Thanks for your help.
Colin