Simp
You are in danger of losing focus on the key issue!
The PTSB owed you money, you didn't receive it, PTSB still owe you!
Those are the simple facts!
Anything and everything else that may have allegedly happened does not concern you. You had no hand act nor part in whatever may or may not have happened. Politely decline to get involved with any third party. This is between you and PTSB only. The PTSB owed you money, you didn't receive it, PTSB still owe you!
Go to the branch responsible and be reasonable (which you have been to date).
Insist that they deal with your only known facts only:
The PTSB owed you money, you didn't receive it, PTSB still owe you! Please pay up!
Follow their escalating complaints procedure.
[broken link removed]
Request any refusals in writing and copy them to the next level...
But only deal with the facts known to you firsthand:
The PTSB owed you money, you didn't receive it, PTSB still owe you! Please pay up!
Tediously boring I know, but focussed!
Best of Luck!