Never, ever, ever, ever, ever, ever eve, and I repeat EVER try and talk with a debt collector. I don't know how much more forcefully I can stress that point.
Debt collectors or Debt Recovery Agents (DRA) in these situations are powerless and their single purpose is to extort as much out of you as possible, relying on the already displayed gullibility of their victims. The DRAs are not a party to any contract allegedly breached by the driver with the PPC.
DRAs work on a no-win, no-fee basis for the PPCs. They use scary language with words like "CCJ" and "bailiff" knowing that the majority of the low-hanging fruit on the gullible tree will simply pay up out of ignorance.
So, no, don't ever expect a DRA to be on your side. In this case, if it is the PPC still chasing you and you have solid evidence you have paid what they are claiming, then take the CAB advice on how to get them to stop or how to defend it should they try to make a court claim.