As far as the contravention goes, the only avenue to explore is when they put the sign up. You were within the bay which has a single sign, on which the suspension sign was mounted, so it was in the right place. The suspension started on Wednesday 7th Jan, you last saw the car on Sunday 4th Jan. The sign should have been in place on that date, so I suggest you submit an informal challenge stating that when you checked your car on the 4th Jan, you saw no suspension sign, therefore request the PCN be cancelled.