In the end, having failed totally to ascertain the whereabouts of the missing stitch, I simply made another one before the blue marker and went on as if nothing had happened. Insouciance is my middle name.
And you can't tell! - the herringbone pattern looks absolutely fine. In searching for the logic, I can only assume that because I somehow or other made a stitch vanish between the orange and the blue markers, creating another one within the same stretch was acceptable to the monstrous garment.
I am about to have to change to a new ball: the Bendigo Woollen Mills balls I use equal four normals, which is why it's taken me so long to reach this
Any of you clever clogs feel like contributing your views on the best way of starting a new ball in the round ...?