Thank you for spending time on this. It does seem illogical for there to be an annotated pdf column if no submission is required, but perhaps 'not required' should be understood as 'possible but not mandatory', which explains why the column does appear, in case you need it.
I'm sorry, in fact I had just discovered the (very simple) answer just as your message came through: it was just a difference in display between my two assignments. I needed to scroll down to the bar to display the extreme right-hand side of the page where I found the Annotate pdf column. It was just not in the same place as in my other assignment.
Many thanks. Problem solved.