Carryfast:
The slower you’re going the greater the speed differential between the truck and overtaking car.Which obviously leaves less time to notice that the car has refused to do as expected in giving way,then reacting to the situation and stopping.
Yes, but another counterpart effect to the truck itself going much slower, is that it changes position and cuts in much slower.
Of course, you can always fail to see a car that hits you in a way that you did nothing to cause. If you’re watching your o/s, and a car flies out of a junction on the n/s into the side of you, you’ll probably fail to see him coming and fail to see the collision, but there’s no question who is at fault for that. You could be completely stationary, and if a car hits you then generally it’s his mistake - you can’t have constant 360 degree vision.
In the OP’s case, the car has collided in an area that the OP should have been watching very closely indeed - even if the car hit him and the OP was faultless (because the car mis-steered and went into the trailer without being pinched, for example), he should have seen the car approach, because with the tractor angled to the o/s he’ll have had a full view of his o/s and beyond in his mirror. But that’s not the narrative that the OP has given - he says he saw nothing whatsoever of a car that clearly has been pinched between the trailer and the kerb.
He’s said the car appeared out of nowhere, and at that place at that time, it’s just not my experience that it can happen. It’s like guys who say they turned a corner and the tree they hit appeared out of nowhere - it asserts something that is not within normal experience (and even if the tree fell in the wind at that very moment, you’d see it happen even if you had no time to react). I’ve had cars move into the cut-in area and cause me a lot of hassle, to the point where I’ve even thought I might hit them because of my speed and course, but I’ve never not seen them.
Realistically the idea of putting the onus on the truck driver to avoid a side swipe situation involving cars overtaking trucks through roundabouts is a joke that contradicts rule 187 of the highway code.
I’m afraid the onus is very much on truck drivers to avoid side swipes in cases where the trailer is cutting in. The normal method of doing that is to obstruct entry to the lane so that the traffic does not come alongside in the first place. Once traffic has come alongside, it’s the drivers duty to give way and stop if necessary.
Obviously, there are conceivably cases where other traffic is behaving so aggressively like a chariot race that the truck is really faultless in a collision, but that’s the exception not the rule.
A similar analogy would be noticing the front of a truck coming round a hairpin bend on a mountain road as you’re approaching it in the opposite direction.Here’s a clue it’s a good idea at that point to stop and don’t even think about entering the corner because you know that there won’t be any road space for you to use when you get there.Just as in the case of trying to go alongside a truck through a roundabout.The result being a collision caused by driving into a foreseeable obstruction across the road. 
I had this analogy in mind myself, but came to the opposite conclusion. If a truck has already come around a right-hand hairpin bend and is starting to straighten up but the trailer is still dragging across the opposite lane, and a car comes steaming along and crashes into it, the car can’t avail himself of saying “the truck was in my lane” - in some circumstances, a truck is entitled to cross lanes and effectively force traffic in those lanes (whether they are adjacent or oncoming lanes) to stop.
But if the truck and car meet at the bend at roughly the same time, the truck (which intends to take the whole carriageway) has to give way to the car - he can’t just say “the car saw me coming, he needed to stop and give me plenty of room” or “he should have known the room I need to manoeuvre”. It’s not until the truck actually begins to cross the white line (which must not occur until the truck driver knows it to be safe and that drivers will have time to react to it), that other traffic can be expected to see what is happening and stop. In difficult situations with aggressive traffic, the truck driver simply has to crawl at snail’s pace over the white line, slowly narrowing the available space until someone loses their nerve - he can’t just swing across and crash and say “they should have seen me coming”.
You cannot assume that car drivers know what a truck is going to do before it does it - that’s the nub of it. A car driver can be expected to see that a truck has partly blocked a lane (leaving a slither of lane, which may become even narrower in the following moments), and not to try and squeeze through. But they can’t be expected to predict that a truck will partly or wholly block a lane, whilst the truck is still proceeding quite normally within its own lane.
