Ex forces PPI claim

I was in the forces from 1992-1999 and during most of this time was stationed abroad. I know I took out several loans with TSB but was unsure if they had PPI or not. Knowing that the claim period is soon coming to an end, I decided to fill in their online form. I had a letter back last week stating they couldn’t find any accounts in my name. I have no paperwork from 20 years ago so thought I would just need to forget about this. However, I received another letter this morning stating:

I understand you have a concern about your PPI and the way it was sold to you. We’ve now identified that you have held PPI on at least one of your loan accounts and we’re currently investigating your concerns. To help us with our investigation, I’ve enclosed a PPI questionnaire for you to complete.

It goes on to state that they appreciate I may not be able to answer all the questions but to provide as much detail as possible. Then “You don’t have to complete the questionnaire, but if you do, it will help us review your complaint”.

I’m really not sure what to do - seen as I don’t know when the loan was! After any advice - should I try to complete it or not? Or should I write a letter stating that I want to know what loans and dates the ppi relates to. Are they obliged to disclose this to me? I was definitely abroad and in the forces so I do think the PPI must’ve been missold due to the sickpay regulations in the forces and the fact that I had signed up to fill service so wouldn’t have been at risk of redundancy etc.

Any help on advice from anyone with experience of claiming under similar circumstances would be very much appreciated. Many thanks