I've been reading several articles on difficult Sudoku puzzles and whether "guessing" is ever acceptable. The attached image shows the stage I reached using only deduction but at this point I could not see a purely logical correct next step. I eventually found the solution by "brute force" choosing one number from a pair and proceeding from there. (My first choice was the wrong one of course).
After posting this question I was introduced to a Sudoku solver, a fascinating insight into strategies I have never considered before. I could follow everything although whether I could manage it with just pen and paper is another matter. The only problem I have is at the step which requires use of the X-CYCLE. I cannot see how you decide that C9 is where we should try 4 as the potential solution. Once chosen I can see how the strategy works but why choose 4 in that Cell over 4 in any other cell?