Sufficient for what? Convincing Parking Eye to cancel the charge? Highly unlikely.
The first bit they will just tell you you are wrong.
The second bit they will deny, they send out hundreds of thousands of PCNs which they believe are compliant.
Third bit they are using POFA to hold the keeper, you, liable, so not relevant.
Bottom line is you didn't follow the rules for the drop off zone and I think trying to claim you were dropping off and not parked for 23 minutes would be difficult. I'm not surprised the land owner isn't amenable given the location.
They will also have detailed documentation of the signage so unless some has gone missing that won't get you very far, especially at the first appeal.
So appeal, it will almost definitely get rejected, get a POPLA code, appeal to POPLA, they will probably uphold the issuing of the parking charge, Parking Eye has a fairly high win rate at POPLA. Spend the next few months getting debt collectors letters, followed by a claim which may or may not proceed to court which you will initially have to defend. If it goes to court then you could try the PCN not being 100% compliant defence, then we might know if it has legs.