Here you go:
Dear London Borough of Brent,
I challenge liability on the ground that the alleged contravention did not occur but even if it did, it must be de-minimis. According to the clock in my car I passed the signs at 2:29, prior to the commencement of the restricted hours, but even according to your video I passed the signs when it was only 2:30:18, so only 18 seconds into the restricted hours.
In light of the above the penalty charge should be cancelled.
Yours faithfully,
Send this online via https://brent.tarantoportal.com/PCN/ and take a screenshot of the confirmation page.
The screenshot below isn't needed now, but will be handy if we have to take this to the tribunal.
(https://i.imgur.com/xPNzB5v.png)