Thanks for the suggestions guys, I should have mentioned I also use a Fuji X-T1 (light leak repaired) and don't get this on those images, obviously leading me to think its not the holder or filter. I have had the exact same issue on a previous 1Dx and after getting that replaced (twice) that lead me to think it was the holder! I normally have the brass knob of the Lee holder at 3 o'clock looking from the back of the camera in either landscape or portrait format and as you can see below the leak moves with the orientation of the camera. To be fair its not always this apparent, I do think its dependent on the direction of the sunlight. In that image it was straight on. Here is another LE of 4 minutes (the one above is 7 minutes) taken with a different Big Stopper but same holder and as you can see its less intense but in the identical place in the frame, the sun in this image was setting off to my right. So different 1Dx's result in same, different Big Stoppers result in same, only common factor is the holder. The 2nd Image here was with the X-T1, again straight into the setting sun but no evidence of problem, 5 minute LE.