Hey Hugh, just from my own limited experience with these cameras, if you let the camera apply too much gain you’re going to get noise. I never found the canon to be that great under dark situations Try to limit the gain to +6 at the max. A couple other basic suggestions to look at (which you’ve probably already done)
1) Turn off ND Filter
2) Open Iris all the way
3) Drag the shutter to 30 (15 if you’re desperate for low light)
4) Buy/Rent the camera mounted LED light kits. Many can be purchased for around 500 and will save you from artifact hell.
Sorry I can’t be more elaborate on this camera. Best of luck!
Kind Regards,
Chris K.