Contravention Code 16: Dollis Hill Lane, Brent London

At the moment, you are still within the discount period. So you should submit an informal challenge based on what has been advised as a case. When they inevitably reject, they will reoffer the discount. At that point you can decide. If you just cough-up now, you are gifting them money.

If you could take pictures of the part of the street where you parked showing any markings or signs or lack of them between the bus stop and the other end that would maybe help.