...and to prove that it can be solved, here is another version sent to me by Arun, a work colleague. You can see that he has used the Excel spreadsheet, which you can download using the link below the puzzle:
The next version came from Christine, who works for Total in France:
The third correct solution came from Solving Sudoku, who is a member of my Sudoku Puzzles group on Facebook. He said: I do it finish. Maybe I'm not careful, so I have to do it many times. I'm sorry. Can you give me max hard? Have a good day. Unfortunately, at the time of writing, I only have one puzzle more difficult than this one but hopefully I should be able to make some more:
