In any event, your contravention relates to a parking place, not a waiting restriction.
IMO, you have grounds for reps: I parked on an unsigned length of road and therefore could not breach any restrictions purportedly related to a parking place. The PCN must be cancelled.
If the authority disagree, they must prove the existence of upright traffic in the vicinity of my vehicle which convey that relevant parking place restrictions apply.
Wait for others.
They won't cancel, but at least you'll test their reasoning, or not!