A few hours later... victory!
I was trying to submit "This Remake Is No Fake Part 1" but was only ever able to select the categories relating to my journal. A bit of Goggling revealed this has been an intermittent problem for a while. What I eventually found was that category options were available for simple text so it was the image at the top which was causing the problem.
So I was able to work around it by
- removing the image
- resubmitting the piece- hey presto all the category options are back
- once the piece is safely in and visible where I want it to be edit it in sta.sh write and re-add the image
- save and bingo the image is there and the piece is still where I wanted it to be