I'm struggling to see why the NTK is not POFA compliant. If InterCity125 is suggesting the wording isn't exactly what's in the legislation Group Nexus won't agree and it's unlikely POPLA will either. You may eventually get a judge to agree but that's a long way off.
there has been no loss to the gym as no payment is needed (only need to enter reg when u visit)?
That's not how this works, the driver entered into a contract with Group Nexus, they are alleging the driver didn't adhere to the rules for the car park, I.e. register, so under the contract the driver now owes Group Nexus. As GN don't know who the driver is and they are relying on POFA and will hold the registered keeper liable. You have four options.
1. Pay the PCN, on the face of it the driver did breach the car park rules.
2. Get the gym to cancel it, usually the prefered option known as plan A. Depends on what sway they have with GN.
3. Appeal on the basis the driver was a gym member but forgot to register, theres an outside chance this will get the PCN cancelled or reduced to £20 (the driver has cost GN real money to issue the PCN).
4. Use the appeal above which will almost definitely get rejected*
A POPLA code allows you to go to the 'independant' appeals service, the next stage in the process.
*This root will most likely go to POPLA next, followed by debt collectors letters followed by threats of court followed by a letter of claim. It's a long winded process if you can be bothered but you will need to engage with the legal process if it gets that far. If you do they normally give up just before they have to pay the court fee. If you don't engage at worst you risk a CCJ in default which will wreck your credit score for 7 years.