The parts themselves cost me €14 for a Focus, its just two pieces of U-shaped metal screwed into the floor between the gap in the back-seats.
What year is your astra? Most cars back to '00 are isofix-ready, having the holes in place on the floor/chassis behind the seats.
edit: If you can fold the seats forward you should see the 2 small holes on either side.
Its a standardised system used for attaching baby seats to the back seat of a car.The isofix baby seat has two clips similar to a door lock that lock on to two u shaped clips that are attached to the car body.A much easier system than trying to remember how the seat belt is routed through the seat........................Much much better!!!!