@janet-9
//although i think the whole accident was caused by the van and car and my braking//
I quite understand how you could view the whole thing as a single incident. However, I think the Police would regard this as two, or even three, incidents.
i) Someone nosed out of their driveway without much attention to the possibilty of oncoming traffic and was struck by the van (we could debate the road markings not being double white lines at this point, to which I say "tractors!")
ii) The truck hitting the out of control overtaker
iii) You arriving at an accident scene (albeit in an instant) and failing to come to a stop because of insufficient braking distance.
I can't remember the last emergency stop I did from 40mph but I might try one on a quiet road to remind myself how much road I need. In the meantime, it's the "2 second rule".
With regard to "instantaneous" occurrences, there is such a thing as "reading the road" which, in my case, includes reading the behaviour of the vehicles in front of me and adjusting what - and where - I am to cover a whole set of 'what if?s'.
In the example of an overtake on a two lane road, I am faced with 10-15 seconds of having no 'escape area', into which I could swerve or skid to avoid one or both vehicles in front doing an emergecy stop (I would be expecting traffic coming the other way, around the blind bend). So I have no choice but to fall back (safe braking distance plus a bit, for comfort).
So, my apologies for rambling but these are my thoughts but I might not have the full facts. Plead guilty to get proceedings over with quickly. If you genuinely think the truck driver did something wrong and you can prove it, then plead not guilty (which, nevertheless risks losing, with legal costs on top of anything else).
Personally, I would be grateful to still be in one piece. Stay lucky!