You have not got the original PCN, not probably a reminder - this is often because the V5C has not been updated, and the reg address is not correct. Suggest you get it out and check - assuming you are the reg keeper (i.e. it's not on lease?). If wrong, then a) you need to get it updated and b) you need to send a data rectification notice to APCOA (in case they do decide on court).
You can send a complaint to them (as reg keeper only!) saying that you didn't receive the original PCN - and that they need to consider as an appeal. The basis is that the airport is not relevant land, they don't know who was driving and you are not obliged, and will not, tell them who was, and therefore they can't enforce against the reg keeper, and must cancel the PCN. APCOA usually roll over when confronted with this.