The penalty is at the full amount so there's no point in paying now, you might as well send something:
Dear Transport for London,
The alleged contravention did not occur. This is because the box junction markings are extended beyond the junction, which is not authorised by the traffic signs regulations, box junction markings can only be placed at the junction of two or more roads. As my vehicle stopped on a part of the box junction markings that is beyond the junction itself, the alleged contravention cannot have occurred. The PCN must therefore be cancelled.
Yours faithfully,
Send it online and don't forget to get a screenshot of the confirmation screen.