Ok problem solved (I hope permanently).
The first photo was taken on the night of the 22nd before I rebaked the desiccant. A straight drop from ambient to -14 deg in one go. You can clearly see frost around the extremities of the image. The second photo was taken this afternoon with the sweat running off me, very muggy. Ambient at about 25 deg. I dropped the camera temp straight down as far and as fast as it would go, down to just below -10 deg (mine must cool more than the stated 30 deg which is fortunate
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As you can see there is only the slightest trace of frosting in the corners (you may need to flick between the two images). There is no way I could have done this before. When the new seal comes from SBIG I'll just keep it in reserve. If this one is working it will be handy to have an unblemished spare
Added an animation