So I was up til about 4am tweaking this puzzle until it was just right. In case you were wondering whether I can do hard puzzles, here's an extra regions puzzle which will push your geometric understanding of the added constraints quite far. Let me warn you now, until you work out exactly what is going on, you aren't going to make much progress. Puzzles like these where there are no freebies at the start, and where every given plays its own crucial role both by itself and in ensemble are very hard to come by, and I think this is probably as good a puzzle as anything I saved for the UKSC. If not better! And whilst I'm here blabbering on, I'm also going to take this opportunity to also point you towards Bastien's wonderful 16 given Diagonal.
#246 Extra Regions Sudoku – rated hardAll puzzles © Tom Collyer 2009-13.