Unfair Parking charge

[Giz 77]

Hi wondering if you can assist, I was parked in a private car park for 13 minutes, i was prepared to pay for my parking but there was no pay and display machine where the large sign saying pay here was. I was shocked to receive this letter and upon looking further there was 3 signs approx A3 size around the carpark saying payment is online only. Last time I parked in there was a year ago and there was a machine so didnt think to much of it. For 1 hour of parking it was only £1 so was more than happy to pay this, any help on how to appeal this would be great. Thanks

[Giz 77]

Photo of letter*