Park Direct UK Uxbridge - Help

On friday evening I went to Uxbridge Civic centre to collect my friend, I couldn't see her anywhere and as there's was nowhere for me to park I decided to turn the car around to leave, to do this I had to go into the road that leads to the gym and as it's a narrow road I had to slightly move to the side to let a car through, did a U-turn and at that point my friend saw me and jumped in the back of the car and I left the place. At no point I parked/waited or left the car.

I've now received a letter from the Park Direct UK charging me £100. The 2 photos show the back of my car with the reverse lights on, on the first photo you can see the back of a car in front of me and on the second photo you see the front of the same car in front of me, which happened when I was letting this car move out. The photos show a date & time stamp as 1st - 17/10/2014 20:53:32 and 2nd - 17/10/2014 20:53:42 but the letter states "Contravention time 20:55".

The reason they give is:
“We believe that a parking charge is payable with respect to the above vehicle for the following alleged contravention: stopping or waiting where stopping or waiting restrictions are in force at a location which had the terms of parking clearly displayed on the warning signs.”

I need to appeal but have never done this before and have no idea where to even start. Can anyone advise/help me please?