Sorry - tried changing visibility of the grade category but it didn't work. I added the assignments to a new grade category and set that to hidden - it had no effect on the display for students. I then tried setting the assignment to hidden in the gradebook and it hid everything including the feedback.
I found a workaround by adding display:none code to my theme - but it removed the information about who graded the item as well as the grade, and also hides any rubric marks completely (which would mean rubrics can't be used to give feedback). I've raised a tracker issue about this as it would work as a solution if the gradingtable items were defined separately rather than lumped together with the same cell variables. Tracker item and the workaround - MDL-36995