I see mine and another initial response described it as a slam-dunk case in favour of the council, and I see no reason to change that. They have re-offered the discount, and their response seems to be in order, so it is your decision whether to pay it, or register an appeal at London Tribunals with the full PCN penalty in play.
Sometimes the skill and diligence of our "ferrets" is not enough to overturn a PCN !!