Thanks for your reply ericsbrother, I might be sounding really thick but I don't understand where the 35 days come from?

We parked at Port Isaac car park and paid £5 for a full days parking. When we returned to the car 2 hours later we had got a PCN. It looks like we may of left a window slightly open and the ticket had overturned. After searching these posts I chose to ignore the PCN and wait for the NTK arrive. I sent an appeal with a copy of the ticket when the NTK arrived stating that they hadn't suffered a loss and that to provide me with a popla code. I've received a letter back today stating that my opportunity to formally appeal has been lost as it should of been done within 21 days of the PCN. It does also state at the bottom of the letter 'please note, we have reviewed your case and can confirm that the PCN was issued correctly, therefore any appeal would have been rejected in any case' Any advice on what to do next please, do I just ignore it or is this company likely to take it further. Also should they have issued me with a popla code.