Thank you for your response. I thought this too but on the website it said:
Successful challenges
If your challenge is accepted or the PCN was issued in error,
it will be cancelled and you will not have to pay. If we reject
your challenge at the full rate you will have to pay the full
fee (the amount will not be discounted).
Rejected challenges
If your challenge is rejected you will be sent details on how
to take the appeal to an independent parking tribunal. This
will be conducted by the Environment and Traffic
Adjudicators (ETA) who are legal adjudicators.
They will reassess the case, either at a personal hearing or
by post.