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the onus has to be on the person doing the [left turn].
To a point, big vehicles have a blind spot. I have seen riders pull off from traffic lights and sit in a vehicle's blind spot for ages. There is only so much the driver can do, if they've indicated for a good length of time and manoeuvred slowly then at some point it becomes the rider's responsibility to not be invisible to the driver as it's not possible for any road user to be completely, fully aware of everything around them at all times. Just like if your rear view is obstructed, the best you can do is slow down as gradually as possible and hope there's no one sat 1 meter behind you. Or if you're in front of a cyclist, indicate left, start slowing down and then they decide to undertake you.
Note, I'm not talking about barely overtaking then turning left here, just normal left turns. @TW didn't say whether s/he was overtaken dangerously or just was safely behind the van as per normal
Edit: point is, if you sneak up into a van's blind spot and get hit when they turn left, that's on the rider
^ this and if they've been indicating coming up behind you between shoulder checks (edited before I get called out for no shoulder checking), what chance do you have?