Free Traffic Legal Advice

I have an outstanding PCN issued by CP Plus T/A Nexus parking. The signage in the retail park clearly states “pay on exit” (no pay machines or exit barriers) so when I was leaving, i downloaded the Nexus pay app to pay for my parking, it reads "you cannot purchase future parking at this location - press continue if you wish to purchase parking that starts now"....i wanted to pay for the time that i had already spent parking not from the time i was about to leave.....as the signage states "pay on exit" then I assume the app would have told me how much i needed to pay ?....the nexus phone number also only gives the option to pay ahead as if i was just arriving.  I emailed CP Plus on arriving home explaining the situation and offering to pay the £2.00 parking, but they said I would have to wait for the PCN and then appeal….appeals to Nexus and POPLA have both been unsuccessful. Advice please….should I pay what is now £170 or await a court date ?