They don't have to show the vehicle parked in situ. The invoice is for a breach of contract by the driver. They don't know the drivers identity and you shouldn't reveal it. Whilst you have not shown the back of the Notice to Keeper (NtK), CEL usually comply with PoFA which means they can pursue the Keeper if the drivers identity is not known.
Why did the driver park on this private land (as notified on the entrance sign, as far as I can make out), without complying with the terms and conditions of parking?