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Fair enough, but their sole evidence is a video which we need to see. 

You can phone TfL and ask for the video which they will send as a DVD, (yes, they are a bit behind the times at TfL, everybody else has the video on-line !).
When they have told you that they will send the DVD, they freeze PCN progress for 2 weeks, (make sure they tell you this !!) then restart it. They assume you will have got the DVD by then, but sometimes it doesn't arrive, so you need to put a date in your diary to contact them again if the DVD hasn't turned up.

Fair enough, but their sole evidence is a video which we need to see. 

You can phone TfL and ask for the video which they will send as a DVD, (yes, they are a bit behind the times at TfL, everybody else has the video on-line !).
When they have told you that they will send the DVD, they freeze PCN progress for 2 weeks, (make sure they tell you this !!) then restart it. They assume you will have got the DVD by then, but sometimes it doesn't arrive, so you need to put a date in your diary to contact them again if the DVD hasn't turned up.

Thank you for this - I have spoken to TfL and they are sending me the DVD with the footage on it. They have said it can take up to 28 days to arrive, but the PCN will be frozen for that amount of time. They said to keep an eye on it online (currently under code SUS20 and marked as on hold) and have given me a reference code I can quote if necessary to prove I had asked for the DVD and the PCN is on hold.

Will report back once I have the footage!

Sigh... I've still not received anything, it is marked as 'On Hold' and yet online the charge value has now increased from £80 to £160. I will phone them again tomorrow.

I imagine I'll have a pretty good leg to stand on to get them at least to reduce it to £80 given that they told me it would not increase while I was waiting, and I have continously chased them?

Phoned TFL again - they have reduced back to £80 and have told me to phone back on the 15th Jan if the footage still hasn't arrived. This whole process is insane. It must have cost them more in staff time on the phone to me, packaging the DVD and sending than it would have to send me the footage digitally.

TfL are in the Dark Ages. I think all London councils now put photos and videos on-line. Quite why TfL send out DVDs when most people's IT kit doesn't even have a reader for them !

Finally have received my DVD from TfL - how exciting.

HERE is the footage link on my Google Drive - weirdly, they have cut the video off before I drive back into the traffic. You can see I cannot fit the bike through the gap by the Hobgoblin truck and the rest of the traffic. I pulled in behind the truck and put my hazards on so as not to obstruct cyclists/other road users from squeezing down the side of the truck. Reset my GPS (you can even see me looking around for my bearings!) and then I continued on my way (not shown as the video ends).

If the Hobgoblin truck had not been there blocking the road, I would not have stopped there.

Any advice on how best to make my representation to challenge this would be much appreciated.

what doesn't help is that the other traffic moves off including the delivery bike with a big box on the back.

Thank you all for the responses - and @HC I appreciate the candidness, as it is always more difficult to be totally objective and unbiased towards your own situation.

I feel that ultimately the reality is a combination of everything that has been mentioned. I would not have stopped if the lorry hadn't been parked there as I would have been able to continue down the road. But as it was, and it was standstill traffic to my right, I made the decision not try try and manouvre my bike through (@mickR I totally understand your point, although my motorbike is a lot heavier and less manouvreable in tight traffic than a delivery moped!) and instead to sit behind the lorry for a moment and re-assess to see if there was an alternative route. The most annoying part is I actually did pull back out into the traffic and continue in that direction, so it was a totally useless exercise anyway!

I'll see if any others have any futher suggestions before I draft a challenge which I will post here before submitting. If it fails and the fine stands, then I can understand that it was a poor judgement call from me. I suppose the irony of the situation is that I stopped there to avoid being in the way of other road users (cyclists, mopeds etc that you see in the video) while sorting navigation, but have been penalised for what is essentially causing an obstruction by stopping on a red route. But hey we all make bad calls sometimes so perhaps this is one of those instances.