OP, let's be clear.
By your account:
We were there to drop off food for our elderly relatives, were there for may be about 10 minutes,
The PCN was served at 14.35 with the time of contravention given as 14.34, this having followed an observation period of 7 minutes from 14.27. The photos show the PCN having been served and a third party having removed this.
The evidence is therefore that you were away from the car at least for the whole of the period 14.27 until 14.35. IMO, this would scupper any defence you might have had regarding 'unloading' as in 'drop off food for elderly relatives'. Had anyone returned to the car before 14.35 then you would have said so and would rebase and undermine your claim that obs only lasted for 1 minute.
So, you're now left with either showing that as the driver entered the zone the CPZ signs did not meet the required standard e.g. missing, misorientated etc, or that the time of contravention is incorrect ' The clock on the CCTV is about 21 minutes behind.'. If you also want to argue that there was not a sufficient obs period then you would have to disprove the CEO's account i.e. notebook entries, handheld computer log, photos and PCN. I doubt there's any mileage in this line of enquiry.
I suppose there's always prospective procedural impropriety, but unless there's a glaring flaw in the PCN nothing short of a c****d-up NOR would assist at which point the full penalty would likely be in play.