I hope you didn't mention anything about signage?
As the NTO (which appears compliant) was issued more than 14 days ago the discount won't be reoffered, so at this point you might as well take this all the way to the tribunal. In order to get there, I would suggest a very basic representation:
Dear London Borough of Harrow,
I challenge liability on the ground that the alleged contravention did not occur.
Yours faithfully,
I would suggest you make this representation towards the end of the month to buy us a bit of time to get the car park order back, but please note that your deadline to make representations is midnight on 1 July and you must not miss this or else all is lost.
Representations should be made online, keep a screenshot of the confirmation page.