When I arrived at the barrier to leave, my ticket was not accepted as it should have been despite everything being in order. The machine gave a me a bill for more then £160 pounds. After contacting the emergency person, I was eventually let out without payment (as none was necessary). We were surprised to experience this malfunction as in the past we have never had any problems. We were given no explanation as to what might have happened. A further point of note - it was quite difficult hearing the emergency contact person as the volume was low and I had to get out of the car and put my head to the machine ( I have perfectly good hearing which is not impaired in any way!).