Grounds that might get the PCN cancelled by Premier Park, nothing obvious, PCN was issued in the POFA time frame and on the face of it you breached the Ts & Cs of the car park. Other might spot something I've missed, was the signage clear etc. (although it was obviously clear enough for you knowing you needed to pay).
You'll get told that entry and exit times don't equate to parking time but as the whole industry is based on that premiss it's a pretty weak argument. You'll probably also be told the PCN isn't fully complaint with all the requirements of POFA as they don't appear to have used the exact wording that the keeper is invited to pay the charge. Also a very flimsy defence which won't win you an appeal with the parking company an is unlikely to win at POPLA.
The usual advice on here in most cases is not about a winning argument (you'll be offered all sorts of very legal sounding defence points which won't generally win appeals), it's about doing enough to run the clock down and hope they don't actually proceed to court. It usually works, not because it has merit but because the PPCs can't afford to take everyone to court so give up. You will need to be prepared to appeal to POPLA, withstand the deluge of debt collectors letters and eventually submit a defence when the court claim is raised. If you do that they will probably drop the case before they need to pay the court fee.