Without seeing what you put in your response to their LoC, it is difficult to advise much more. It seems that you may have referred to some CPR points that are not relevant to the small claims track.
There is no real value in responding to Gladstones at this point. Your defence is already filed and it squarely raises CPR 16.4(1)(a) non-compliance. Their CPR 31.14 reply comes after that, and on small claims CPR 31.14 doesn’t strictly bite anyway. All you really need to do now is:
Keep their letter as an exhibit. It confirms they have not provided the original NtK, only a reminder, and that they propose to “supply the core documents (PCN/NTK…)” later in their evidence bundle.
When you come to your witness statement, you can rely on this: note that, despite a specific request, they failed to disclose the NtK pre-hearing and therefore have not, to date, evidenced any PoFA-compliant notice capable of creating keeper liability. That goes both to conduct (PAPDC / disclosure) and to the substantive PoFA point.
Engaging in further back-and-forth now is unlikely to produce the NtK (if they had it readily to hand they would have enclosed it) and will not change the pleaded issues. Save the point for your evidence and submissions.