Although dewing and the desiccant sounded the potential problem, it turned out to be something else.
As I dismantled the camera off my telescope to prepare to dry the desiccant, I noticed the fiield flattener I was using on my refractor looked a little dirty. However, when I tried to clean it it seems the dirt is caught between layers of the flattener I cannot reach. See image.
Oddly the appearance of this dirt varies and a stable room temperature looks like it completely disappears. Must be at its worst when its super cooled ?
So I redid my flats with the field flattener removed and the images are all fine and do not change over time. So no need to dry the desiccant.
However I have had to send the field flattener back to the company I bought it from to be professional cleaned.
Still very grateful to you for your advice though.