After an extended delay, Southwark have responded this week and have cancelled the PCN.
Quite amusingly it states the following:
I refer to the Formal Representation you have made against the issue of the Penalty Charge Notice xxx.
I have examined the facts surrounding the issue of the PCN and cancelled the penalty charge.
Please be advised the PCN has been cancelled due to an administrative error unrelated to the content of your representations and should not be taken as precedent. The cancellation of this penalty charge has no bearing on the validity of any other PCNs that may be issued to you in future.
In this instance, as the PCN has been cancelled, no further action will be taken in this matter.
Yours sincerely
Southwark Parking Services
Not sure how much of this to take at face value but will take the win. Thanks to all, especially Hippocrates.