@moctey send this and let's see what they say:
Dear London Borough of Lambeth,
I challenge liability for PCN LJ25786740 because the notice mis-states the time-period mandated by paragraph 5(2)(a) of Schedule 1 to the London Local Authorities and Transport for London Act 2003, this should be 28 days from the date of service of the notice rather than 28 days from the date of the notice.
It follows that the penalty charge must be cancelled.
Yours faithfully,
Also, are you able to go back to the location? If so, please find point on the northeastern side of Cornwall Road that is in line with the frontage of 123 Stamford Street: https://goo.gl/maps/HGXFKy2oQGHfKkXM9
Then, measure a distance of 2 metres from that point to the sign (which is the sign you passed on the left in the video).
If the distance between the first point and the sign is less than two meters, Lambeth can't enforce as the sign would be a TFL section of the road.
Lastly, you might want to redact your postal address form the image of the PCN you've posted above.