This is open and shut, send this:
Dear London Borough of Waltham Forest,
I contest liability for this PCN on the basis of a procedural impropriety. The PCN is required to state that payment must be made before the end of the period of 28 days beginning with the date of service of the notice, but the PCN in this instance wrongly states that payment must be made within 28 days beginning with the date of the alleged contravention.
As you allege the contravention took place on 18 October and the PCN was served on 31 October, you have cut the 28 day period short by two weeks.
As the PCN does is not substantially compliant because it does not convey the 28 day period that it should, there has been a procedural impropriety. This means the penalty charge must be cancelled irrespective of any other matters.
Yours faithfully,
Obviously you want to send this via the council website, and keep a screenshot of the confirmation page. If they reject we'll sort out the tribunal appeal for you.