Firstly, Katherine Road is a long road, so it is essential you give us a GSV link telling us exactly where your father parked.
Secondly, as you are at NtO stage, it is a no-brainer to submit reps, and, if necessary (if we think it is viable) take them to London Tribunals. It is likely that if you submit reps stating you never received the PCN, they will re-offer the discount. Certainly advice is not to pay at the moment, until you have had a response to representations.
As for non-receipt of the PCN, this is very common with PCNs served to the vehicle, so not grounds for an appeal, unless you can definitely prove it was not served. With parking PCNs it comes down to who the adjudicator believes, using the civil law test of "on the balance of probabilities". Photos are not even a legal requirement.