Which bit of this advice was not clear?
You can safely ignore all the useless debt recovery letters you are now going to receive. Debt collectors are powerless to do anything except to try and intimidate the low-hanging fruit on the gullible tree into paying up out of ignorance and fear.
Come back when you receive a Letter of Claim (LoC). All debt recovery letters can be shredded and used as Hamster bedding for anyone cares. We don't need to know about them.
ZZPS is a powerless, bottom-dwelling firm of debt collectors with absolutely no power to do anything. Treat them as the equivalent of brown smelly stuff you avoid stepping in. You can also ignore the next set of letters you will probably receive from GCTT, which is just another knuckle dragging moron sitting at the desk next to the anal orifice who sent you that letter.
An LoC will come from a bulk litigation firm and will say "Letter of Claim" or Letter Before Claim" or something like that on it and it will give you 30 days to pay, not the 14 days that these useless cretins give you.