Many thanks for your assistance.
I have taken all the points raised here (word for word in many cases) and have drafted the below:
Dear sir/madam
As can be seen on the video evidence, my vehicle did not enter any box junction at 3:29, in fact, the video evidence confirms my car had already entered the box junction at the start of the recording (3:28). Unfortunately the video evidence does not show the time of entry, which must have been before the start of the recording.
Furthermore, the video evidence shows that my exit is clear on entering the box. Space large enough to accommodate the vehicle can be seen beyond the junction markings.
My vehicle did not stop for stationary vehicles. I was forced to stop by a moving vehicle (the bus) cutting across the lane in front of me. The bus and my vehicle became static at roughly the same time. Also, in order to avoid an accident, I was forced to give way to a motorcyclist trying to squeeze through a gap in front of me.
Once stopped, the video evidence clearly shows that there was enough room ahead of my vehicle to clear the box, however, safety concerns had precluded me from making use of this space. I did not make use of the space ahead as a precaution against the bus cutting further over once the traffic began moving again.
As no contravention of entering a box occurred at 3:29, and as on entering further into the box, enough space to accommodate my vehicle can be seen beyond the junction markings, the contravention alleged on the PCN did not occur and the penalty charge must be cancelled.
