@John001 I'd go with something very simple and to the point, based on the issue identified by MrChips:
Dear London Borough of Lewisham,
I challenge liability on the ground that the alleged contravention did not occur. This is because there is a prohibition on entering a box junction if the vehicle cannot clear the box junction without stopping, so the contravention occurs at the point of entry.
The video in this case shows the vehicle entered the box junction at 1:19 pm but the penalty charge notice alleges a contravention at 1:20 pm, so while a contravention might have occurred the contravention stated on the face of the notice did not.
It follows that the penalty charge must be cancelled.
Yours faithfully,
Send this online and make sure to keep a screenshot of the confirmation page.