At the same number of attempts as the answer to 11ac I have finally completed this!! My problem was an idiotic assumption about what the 3rd digit of 1ac must be. What irks me a lot is that my answers in the 2nd row and 2nd column were not a fluke, as I previously thought, they were in fact correct. I think once you have messed up a mathematical you need to go for a long walk and start again, rather than starting again immediately whilst being unable to erase previously incorrect decisions from a disintegrating brain. One simple mistake always leads to failure.