It's simple, you must be able to stop in the distance you have in front of you. It doesn't matter what the car in front is doing. You have to be prepared for that. It's called driving.
Keep left arrows, then a solid white line on marks side, a blind brow, then solid white lines on both sides, then junction sign and diversion signs. The brow should have caused the Clio to back off long before the junction. What if there had been a queue of stationary cars.
Mark's driving is irrelevant the Clio driver should have been preparing for potential hazards long before the junction.