Yes I rang insurance today and confirmed that if someone hits you from behind and then pushed you into the vehicle in front, the guy at the rear is responsible for all cars and only he pays excess if he is identified (so chase him down if he tries a runner).
However, if you hit a car and then someone runs into the back of you after that initial impact, you pay excess and you are responsible for the car in front and you front while the guy who crashed into you also pays excess and is responsible for the rear of your car.
The insurance guy said that if there are no witnesses in multi car nose to tail crash, which often happens at night in low traffic situations, that is where a dash cam can be very helpful. The video documents the sequence/timing of the impact events and thus who pays excess
So all good, thanks