Entering a "O" instead of a "0", or vice versa is common, and there is a robust appeal for that based on the font that must be used for a numberplate but a "5" instead of a "6" is different. However, I think you still have a good case.
Wait a bit for others to comment. You have the right in law to take them to the adjudicators, but must, of course, risk the full penalty if you do.