As this is a postal PCN, all representations are formal, so there is no discount to offer, as is normally done when rejecting informal challenges against parking PCNs.
There is no statutory guidance for councils, and TfL on the enforcement of PCNs in the London Local Authorities & Transport for London Act 2003 unlike the Statutory Guidance under the Traffic Management Act 2004, but I would suggest that this clause in the said statutory guidance surely applies to any councils enforcing PCNs under the penal lowers they have been granted, even it the applicable Act is not the TMA 2004:-
An authority has a discretionary power to cancel a PCN at any point throughout the process. It can do this even when an undoubted contravention has occurred if the authority deems it to be appropriate in the circumstances of the case.
Under general principles of public law, authorities have a duty to act fairly and proportionately and are encouraged to exercise discretion sensibly and reasonably and with due regard to the public interest. Failure to act in accordance with the general principles of public law may lead to a claim for a decision to be judicially reviewed.
Enforcement authorities have a duty not to fetter their discretion, so should ensure that PNCs, NtOs, leaflets and any other advice they give do not mislead the public about what they may consider in the way of representations.
They should approach the exercise of discretion objectively and without regard to any financial interest in the penalty or decisions that may have been taken at an earlier stage in proceedings.
Authorities should formulate (with advice from their legal department) and then publish their policies on the exercise of discretion. They should apply these policies flexibly and judge each case on its merits. An enforcement authority should be ready to depart from its policies if the particular circumstances of the case warrant it.
It seems to me that Havering have fettered their discretion by declaring that they will never re-offer discounts when considering representations. I suggest you formulate your reps to London Tribunals along the above lines.
There may also be a "technical" appeal you can add on which is not related to the contravention, but to council mismanagement of the enforcement process. Expertise on this is with our Hippocrates who will see advising on other threads.