NCP parking ticket

Hi I recently parked my car in a NCP private car park, I paid for a ticket at a machine for 30mins. I did what I had to do then returned to my car only to find I had a parking sticker pouch stuck on my drivers window. On inspecting the ticket it said I was in violation because I did not display a valid ticket, and the time stated on the parking ticket said 09.28. I looked at my ticket which I had purchased from the machine earlier and noticed it was not due to expire till 09.30. Right away I looked about the large car park area to challenge the warden who issued this ticket to complain, but did not see anyone. I mentioned my annoyance to a nearby driver and told him my story, but he pointed out to me that I had used the wrong ticket machine. I had infact used the councils machine and not NCP's own. I remarked I thought all parking machines were the same as they were lots placed around the perimeter of this car park I used, and the only reason I used this machine is because it was the nearest machine to me which had less people queing at. My sister is the owner of the car and was annoyed when I told her of the parking ticket, she wants me to pay it for fear of getting a bad mark against her name. Can anyone advise if I should pay this fine or ignore it. Thanks in advance for any help given.