Hairless..
As the reversing car, it is your resposibility to ensure the way is clear at all times during the manouvre.
However, it is normally accepted that if a car is already moving out of the space- say half in, that any car wanting to proceed past, should stop and wait, and that it would be reasonable of you to expect them to have stopped when they saw you.. But you should also have stopped when you saw them.
Depending on how far you were out of the space- ie, where they already behind you when you reversed out, or were you already reversing out and they went past, you may get the insurance claim to be a partial fault rather than a full liabilty.