Thanks Royston
BUT if this is the case then 10a "type of parasomnia, more commonly called sleepwalking" to which i have the answer "somnabulism" and this fits with all other down clues is wrong.
i also thought of "inro" -a small japanese ornamental case, but then "retina" is wrong the clue for this is "the light sensitive membrane at the back of the eyeball ..."
so i'm back to -N-I
it's the last clue and driving me to distraction.