Thanks rrobbo, shall wait to see !!
Yes, the entire grid should be full before the blacking out, though I confess I hadn't solved 3 clues in the SE quad by the end - as their answers' mutually checked and unchecked letters were blacked out it didn't matter !
9d is the clue to HYADES, there is no entry at the top of the penultimate column
18a does begin with J and is a 6-letter slang word often preceded by 'rubber' ('protective' in the sense of against disease/conception) less the usual stately abbreviation, 'office' being a US slang synonym of the answer (also slang !)