When stationary in the air, does the Mavic use its optical sensors to help it maintain position?
I'm wondering whether it's these which give it the advantage over the Karma which I've seen drift in a lot of videos.
Or is it simply that the Mavic uses GPS better?

Yes, that is a primary purpose of the two cameras and sonar on the bottom (the VPS or Visual Positioning System).  As long as it can lock onto a solid pattern on the ground, it will be rock solid (freaky).

If you are less than 10m then the VPS is more accurate than the GPS.  The limitations are on the contrast of the ground surface and the lighting conditions.  Less that 100 lux will be an issue.

Well yes the VPS system keeps it positioned well as above, but if you watch a hover test with Phantom 2/3/4s they tend to pretty precise anyway. DJIs Naza or GPS systems have always been very accurate compared with others.

DJI make the best kit about, no doubt about it ... they just do customer service badly ;)

Thanks everyone.  This is probably the reason why the Mavic is so much better than the Karma at any kind of tripod type function.