Not sure if you got an answer to this or not, but after the 56 days, the PCN has to be waived automatically. Same thing happened with me with the TFL for a yellow box violation, and it went over the allotted time to reply on their end. So even if you are in the wrong, they have to waive it.
Usually you are contacted via letter or email to confirm this. If they don't waive it and try and charge you, just read back to terms and conditions about the 56 days, they shouldn't have a leg to stand on.