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This is a blatant contravention, if
@CyclingMikey had caught you you'd be at risk of points on your licence for doing something like that. So we definitely cannot challenge the contravention.
In terms of getting the PCN cancelled, two aspects come to mind. The first one is that by giving information about the date of service right next to the payment deadline, the authority is giving somewhat ambiguous information.
The other aspect is down to who the keeper, what's your connection to the LLP that the PCN was issued to?