This is best approach IMO.
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Challenge on the grounds of contravention did not occur.
I am challenging the PCN because I was engaged in the exempt activity of unloading, namely delivering multiple packages to the Marylebone Project at 1-5 Cosway Street, which houses this women’s hostel and homelessness service.
To supplement the project's work, families from north London prepare approximately 400 food packages every 6 weeks. Given that my driving skills exceed those of my cooking, I'm designated to deliver these loads which I do regularly, almost invariably on a Saturday.
Until the day in question, I have parked on a yellow line without issue.
On the day of the PCN I delivered the food packages plus bags of clothes donated by women from north London but could not find a space close by other than in the parking place which I mistakenly thought was subject to the same restrictions as the yellow lines.
I probably made 10 trips to/from the hostel and presumably it was during one of these that the CEO issued the PCN.
I understand that such unloading falls under exempt activity, and enclose a note from the hostel confirming the delivery.
I trust this will be accepted by you and look forward to early confirmation of cancellation of the PCN.