Thanks all for your advice.
I have not submitted anything to the tribunal yet, though hope to do so later today. I received advice via PM to email my appeal, not to post.
The letter I plan to submit is the sheet that is headed 'Details of appeal'. My original representation is the last picture of the saved representation on my screen.
I have added detail to the screenshots to describe the sequence and stage each error occurred. The problem rapidly got progressively worse to the point even the error webpages wouldn't work.
I did originally get to the vehicle details page, and entered the correct details, but this point is where the first crash happened. I couldn't progress any further, and then couldn't to the details page, and then couldn't get any page at all. This was right up to the deadline that night.
TFL have put a lot of irrelevant paragraphs in their rejection letter. Probably because they are trying to reject something that they shouldn't, and their letter generating software doesn't fit the situation so they've entered various other things instead. I am particularly amused about them claiming to have done a thorough financial 'investigation', to conclude that they can't find any payment from me, having been told that I couldn't pay them because their website was not working!