It’s not a fine, it’s a stupid and unenforceable invoice for payment on the basis that you drove onto the land, agreed to the terms and conditions on the signs, and parked, then left again all within one minute.
There’s probably a formal appeal but, personally, I’d be telling them to stop being stupid and there’s no chance you’ll be paying them a penny, and that they’re a bunch of incompetent money-grabbing fools.
If I send you an invoice for spending an hour doing your gardening whereas in fact I spent one minute removing a weed, you wouldn’t call it a fine, you’d tell me to get lost, wouldn’t you?