It is PoFA compliant but not fully. That won't make any difference to an appeal as they will reject any appeal. The NtK does not invite (nor any synonym of the word) the keeper to pay the charge which is a failure to fully comply with PoFA 9(2)(e)(i).
Also, the NtK does not specify the actual contractual terms breached. It simply States that "the vehicle remained at the site for longer than the free parking period". There is no evidence of what the free period is.
You can appeal and wait for the rejection and a POPLA code and then appeal to POPLA and throw the usual "kitchen sink" of points for the operator to rebut but I would wager that POPLA will reject also. Good luck though. I would love to be proved wrong.
In my personal opinion, this will go al the way to a county court claim being issued. That is a good thing because a truly independent arbiter, a judge, is best placed to decide whether you owe GN a debt or not.
If it does go to a claim, there is a good new "short" defence coupled with a draft order that should see it quashed. The only caveat is whether you are up for fighting this all the way with the very remote risk that if it all went south for whatever reason and the judge decided that you sis owe GN a debt, it could cost around ~£200 all-in, which would still be less than what they will be claiming. There is no risk of a CCJ because if paid in full within 28 days of judgment, there is no record of it in your credit history.
The most likely outcome is that you resist any debt collector demands with the knowledge that they are powerless to do anything and can be safely ignored and wait and see if they actually do issue a claim. They may or they may not.