Hi guys thanks for your answers I'm currently solving my sample pool of hard puzzles now, some more testing needed but hopefully its a complete solver.
Thanks so much, it was pointed out to me that row 8 also has a naked quad in it so now I've now coded naked quads and naked triples I'm now solving 9/10 puzzles from my sample pool, the last puzzle is ...
I'm making a sudoku solver which can only use techniques and not brute force/backtracking, I have a number of techniques implemented in my program but I'm kind of stuck on what technique ...