Blimey! Another endgame that was beginning to do my 11a in…
I thought there was an "extra letter" in 29dn, giving a Down Clues phrase of 8,6,1. Since this "extra letter" could be found in the 6 of the completed grid, I thought for a long time that 8 meant this cell should be left blank. Then I tried to replacing 27 with INSANITY which gives real words… but, as someone famously almost once said, "There issa no INSANITY clause!"
Even after being put right by all the helpful hints from you guys and completing the puzzle, I still can't see a synonym of 8 in Chambers that isn't the opposite of what is required!
Roll on next weekend.