Can you evidence that you made two visits? Do you have any evidence that your car was elsewhere that day when they claim it was in the car park.
If you're adamant you were not in the car park for the duration they claim, it raises suspicion that just taking G24s word that they have only evidence of a single entrance and exit for that day is mendacious and that they have tampered with their system and or they are lying. By tampering, I mean they deleted the orphan images.
Their system is supposed to be tightly controlled with logs of every access to it. The DVLA will have asked them for evidence that they have only a single entry and exit for the vehicle that day and they will have accepted G24s word for it.
It also begs the question why G24 decided to cancel the PCN if they had no orphan images. As Marcellus said in Hamlet... "Something is rotten in the state of Denmark".
So, if you have anything, even a witness that could verify that you were elsewhere with your vehicle at the time they claim you were still in their car park, you should escalate this back to the DVLA and the IPC.