Reading your narrative, your circumstances come under what is referred to as "mitigating circumstances", i.e you were not permitted to park there, but mitigating circumstances, ( distressed baby in the car who needed changing and no other parking locations), compelled you to do so. Of course there may well be other, technical issues around the PCN, the parking location etc that would also be grounds for cancellation of the PCN, and others on here may spot one. For instance there is no telephone facility on the PCN for payment of the penalty. It could be considered that this is discriminatory, and therefore grounds for cancellation.
The problem with "mitigating circumstances" is that only the council can cancel a PCN if they accept these. An adjudicator can only accept a set of legally defined circumstances, and mitigation is not one of them. In addition to this, councils are renowned in London for their venality and rapacity and usually reject all informal and even formal representations because they know from experience that most people just cough-up. Having said that, don't be disheartened, because it is always worthwhile submitting informal representations to a PCN, and especially in your circumstances. Sometimes somebody in a council with at least a smidgeon of humanity will accept such representations. So submit reps on the compelling circumstances that essentially forced you to stop there, and post-up their reponse when you get it. Do mention your conversation with the CEO when the PCN was issued, as this should be in their notes.