As the law stands (Beavis v Parking Eye, Supreme Court, November 2015), you have to pay. But the Beavis case was a straightforward overstay and the ruling played into the hands of the parking enforcers who are using it to enforce all kinds of unfair rules. Cases like yours also need to be taken to The Supreme Court in an effort to get rulings that are fairer to the public. But if you are (understandably) not prepared for this, all you can do is pay.