Thank you. Any salient points I should mention?
In my view you don't have a defence (on the facts) as IMO you were clearly parked more than 50cm from kerb (see my post above).
However it is Greenwich who need to make the case of your contravention.
Personally I would take the discount if it is still available.
If it is not - then I would appeal along the lines that H C Andersen suggests, and put them to the test to proving otherwise.
Then hope that Greenwich make a hash of making the case that the distance was greater than 50cm, or fail to provide an evidence pack, at all, to the tribunal. (The latter isn't uncommon for Greenwich).