Get an informal representation (challenge) delivered to Harrow before the 3rd August, before the discount window expires, and they will re-offer the discount if they reject you. It says so on the back of the PCN. That won't cost anything if you submit on-line. So you might as well have a punt as you have nothing to lose and £70 to gain.
Unless someone comes up with a defect in the PCN wording, you'll have to plead mitigation, polite and contrite.
Something along the these lines.
You're used to different restrictions in the adjacent bay where you normally park,
I was mortified to find your PCN on my car,
you believed it would be the same restrictions in this bay,
I didn't intend to evade a parking charge,
terribly sorry,
I made an honest mistake,
it could happen to anyone,
I learned my lesson and I won't do it again,
it's my first offence,
please use your discretion to cancel the PCN.
And so on in that vein. Whatever you can come up with.
Are you a Harrow resident? If so, mention that as well.
Post up what comes back, there might be a fatal error in the rejection.
