'The original letter did not state the vehicle was parked in the drop off zone
Yes it did:
'Location'
Russells Hall Hospital Drop Off'
Their ONLY evidence as to the location is the photos.
Do these show on the balance of probabilities that you entered and stopped in the 'Drop-off' zone? If not, then IMO absolutely carry on.
A POPLA assessor's first consideration is whether a parking charge is owed, to determine which they need evidence.
Theirs is the photos and copies of the signs at the location and the Ts and Cs of parking. But the latter aren't the issue.
Substantially, it would appear that the photos do not prove that you were parked in the Drop-Off zone unless these are from cameras which cover ONLY the drop off zone and there is no other exit from that zone.
Circumstantially, you have payment of the correct fee for parking in the general carpark area. On this point, I refer to this comment in (what you've posted of) their response:
the drop-off car parking space has a 15-minute time limit, and payment is not permitted.
Which suggests that payment facilities are not provided.
So, what are they saying....that you drove in, parked for the stated time, exited the car park to find a payment machine elsewhere, paid, returned to you car and then left. Or perhaps exited past the camera, returned on foot to pay then left?
The more you think about it, the more preposterous it sounds!
Anyway, pl post their actual response and confirm the details of the drop- off point - apologies if this is embedded in other responses.
I agree with others, carry on. But it's your choice.