Thanks so much for the help everyone.

This is PP2012 so there's no IDL options, but your hint did lead me towards the IBL light which I've turned off, another light had depth map shadows, and between those two changes it does seem to be fixed. Was excited by the 'threads' suggestion too, but that had no effect in this case. It must be a change I made in the scene which triggered it, because the first thousand frames rendered with zero artefacts.

Thanks again, nightmare having that kind of thing happen near the deadline for a video project! I need to get my work to buy the latest PP really, perhaps if this video goes well, they will :)