After some experimentation I have found the artefacts disappear if the “Image Scaling” preference is set to “Bilinear” – The two higher quality settings “sharper” and “smoother” both exhibit different edge artefacting.
FYI I am working with 5120 x 2060 RED footage, in a 2560 x 1080 timeline, with Output Format set to 1920 x 1080 (so outputting 1080 with black bars).
Interested if anyone else has experienced similar problems – would love to know if its a bug, a known issue, or a problem specific to my workflow?