Having reached the same crunch point as George, I managed to miss the obvious point that Box 1 "owns" the 6 in column 1 , so r5c1 can only be 5. So in a total fury I chose to apply the old rule "don't ...
Very hard indeed. Part of my problem in understanding the discussion thread for this puzzle is that I'm unfamiliar with the numbering scheme. R and C for row and column are clear but are columns left ...