What has your management said about getting this cancelled? Have you tried asking Sainsbury's to get them to cancel the Parking Charge Notice (PCN)? This is called Plan A.
Where is it actually documented, this convoluted way of using overflow parking outside of the hospital car park area with different delineated permit areas? Relying on gossip from colleagues is not an approved method of notification. If it I an approved procedure, it must be documented somewhere. Find out where. Your line manager or, if you are a union member, get them to assist you if getting to the bottom of this.
Please show us the Notice to Keeper (NtK) you received. Both sides and only redact your personal info, the PCN number and your VRM. Leave everything else visible, especially all dates and times.
You have received the PCN as the known Keepr of the vehicle. ECP have no idea of the drivers identity and it should remain unknown to them. Only refer to the driver in the third person. No "I parked here or there", only "The driver parked here or there". There is no legal obligation on the Keeper to identify the driver to an unregulated private parking company.
No one who is here receiving advice and following it pays a penny to ECP.
For now, please answer the questions and show us the NtK.