Considering the contravention only occurred 1 minute into the restricted times, I’d say you have a very strong case in this situation already, and can win possibly alone on De Minimis, as a car clock cannot be expected to be atomic and display the time exactly. The Act they use to enforce bus lanes is also very advantageous, as many appeals are won alone on Schedule 1, Paragraph 7(6) of the London Local Authorities Act 1996, as the authority does not attend which means an automatic win.
Currently, I believe an appeal solely based on De Mimimis will be good enough to the authority, the rest can be left for the Tribunal if they reject.
Let’s leave time for others to comment.