I am fairly new to Sudoku and have hit a brick wall with this one. My colleague said it can only be solved with a guess and seeing how far you go. However, I agree with many on this site and you should not need to guess…it is a mathematical problem so no need to guess. I believe I have gone as far as I can, but welcome someone telling how to get one more number without guessing. Thanks.
(The blue cells were pre-completed.)