Matinee renders at quarter res then upscales

When I launch a Matinee render on my level, It opens a tiny window (480x270) and saves the sequence as 1920x1080 JPGs looking like a blocky, upscaled version of the tiny window. I have now been trying to fix this for days and am just a bit confused as to where this comes from.

I suspect my issue is linked to AA after reading this, but I am not sure after testing:

Observations:
UE 4.11.2

  1. In the same project, but in another level, I can output full HD.
  2. when migrating my level into a fresh scene, I get the same issue
  3. when creating a new matinee on the same level, I get the same issue
  4. I have disabled AA on all my cameras and in: Edit/project settings/rendering
  5. all my cameras have a “screen percentage” post process setting set to 100%
  6. I have used both the level blueprint or the “auto start capture” tick box in matinee, same result.
  7. I also get a lot of crashes when launching “capture movie”
  8. I originally managed to output at 1280x720, but it all went wrong after I switched to 1920x1080. However, I may have changed other settings between the two, as I wasn’t tracking each step then.

I have also updated my Nvidia drivers (GTX 970), but since I can render OK on other levels, I assume it is linked to a setting in my level.

Thanks in advance,
Mat