Here is my suggestion (you'll have to remove the spaces from the youtube URL, I had to add them as otherwise the forum video integration was hiding the text):
Dear Manchester City Council,
I challenge liability for PCN MC15861930 on the basis that the alleged contravention did not occur. When I approached the bus gate I saw the signs and attempted to reverse out, however the driver behind my was blocking my path so I was effectively trapped. The council has placed a traffic island at this location such that it is not possible to take an alternative route if drivers in close proximity to one's rear refuse to reverse.
As an illustration of this scenario, I refer you to the video at https://www. youtube .com/watch?v=4KxRKgIx0hE which shows two drivers executing exactly such a manoeuvre, but as that video shows this is contingent on the driver at the rear being willing to reverse.
I could not ram my way out nor could I reasonably be expected to get out of the vehicle to start remontrating with the driver of the car behind mine, as that could easily escalate into a road rage incident.
In the circumstances, I was forced to proceed by circumstances beyond my control and for that reason, the alleged contravention did not occur.
Yours faithfully,
However before you send that, can you please confirm what else you've sent to the council? This appears on the case history:
(https://i.imgur.com/Mo8VR7H.png)