As per Hippocrates, moving traffic contraventions don't engage the TMA regs quoted neither do they make provision for 'procedural impropriety'.
BUT I was just about to pay the charge (this is the last day at 50% discount as it is within 14 days of the date on the PCN) but Southwark have given an incorrect web address to make payment. When you enter the one they provide on the PCN you get a 'page not found'.
So did you pay and if so how and at what rate?
IMO, if you were actually prevented from paying at the discount then these grounds apply:
(e)that the penalty charge exceeded the amount applicable in the circumstances of the case.
The circumstances being that the payment to (vii)the address to which payment of the penalty charge must be sent; was not possible.