I am rather astonished at reading your narrative, because
right behind your car by the parcel boxes is a sign stating you have to pay for parking :-
https://maps.app.goo.gl/6K4nSy1pHNQKxnKj8It is your duty as a motorist using this off-street car park to look for any possible restrictions or payment requirements for using the car park, yet you did nothing. Sunday is no longer a special day, and most car parks require payment, although we have been lucky here in Crewe and Nantwich as Sunday parking is free here, but not for long !
I have to say, I cannot see any credible appeal argument, but there could be a possible collateral challenge based on council management of the enforcement process, but basically you look bang-to-rights on the contravention itself.
Advice in cases like this, is for you to submit a non-confrontational representation admitting your error and asking for forgiveness, adding in anything you might consider as mitigation. For instance are car parks local to you, free on Sundays ? If you submit reps within the discount period, they will re-offer the discount. Post up their response here when you get it.
