'He didn't affix any PCN nor was prevented from doing so but then I received a PCN by post!'
Whether he was prevented or not is your opinion. It might be right or otherwise, but it's not an objective fact and IMO you need to break down what transpired into smaller events.
As regards 'he said I had to park at rear which was obviously occupied at the time.' pl post a GSV of exactly where you were.
My inclination is to say that as loading is an exemption(where is your receipt and did you pre-order etc...you haven't yet dealt with this point)then whether the exemption applies is the key. I don't think that the exemption can be disapplied by a CEO simply because in their opinion you should have parked off-street. There are some off-street parking areas around my way which I wouldn't drive into if you paid me, their surfaces are a hazard.
You would also add that on the face of it the PCN has been served out of time because more than 28 days have expired between the dates of contravention and service. It is NOT your task to reason why, it's the authority's burden to prove that they were entitled to serve 'late' ... because it's hired and they notified the keeper that reps to the effect that the vehicle was hired were served less than 28 days ago. Proof on their part(copy of NoA to hire company), not assertion.
