from ZZPS, GCTT or their ilk, is not required. They are not a party to anything. They are totally powerless, irrespective of what they put in their letters. There can be nothing in a debt collectors letter that has any weight behind it, whatsoever.
We're not talking procedural effect, we're looking for indicators in the wording that what would be argued is already accepted by the creditor, on whose behalf, of course, ZZPS act. And as agents of the creditor whatever is included can be attributed to the creditor. The fact that the letter might be extra-procedural does not render it unnecessary to review, IMO.
OP, PM a copy to me if posting causes problems.