You've posted the Notice to Owner. Any reason for not submitting reps against the PCN ?
Anyway, the PCN is down to your error, so really, unless the PCN contains fatal errors, you can only ask for mitigation and also point out that the council did not lose any money as you did pay to park.
If they refuse, will it be worth going to London Tribunals ? Well, maybe, but if they re-offer the discount probably not. If they don't, it's a no-brainer.