Thanks Dave,
I had come across your Stock Answer #1, and hadn’t thought it through too much… Then I realised I’m using XDCAM HD & EX, which I think are based on mpeg-2?
Strangely I don’t have these sorts of problems when re-importing HD renders of the final project, only the very big canvas with alpha channel…
In any case that makes a good deal more sense now. Thanks for your help. Unfortunately since I have vast amounts of archive footage to deal with, re-compressing it all isn’t an option this time round, but it’s helpful for future reference. Thanks again.