Very strange. Retrospectively applying affordability criteria to a loan is one thing if it is the government doing it. However, this suggests to be a company that bought the debt book of the government, they certainly won’t have bought the legal right to retrospectively apply affordability criteria.
Letter to the current Minister and s separate one too your MP to make them aware of what’s happening and to ask the Minister if the Minister gave the debt collection agency authority to retrospectively apply the criteria.
As for opening the debt, I seem to remember a six year limit of liability on debt if you have not been asked for it nor admitted to it. They appear to be trying all they can to start that clock ticking.
A letter to them denying the debt and explaining coercion for fol signing a repayment plan should suffice. Then, as others have said, ask them to prove the debt and make them aware you have written to the Minister. Also tell them you are considering action against them for fraud on their part. The accusation alone will have them running to a lawyer.
Nearly forgot, report the suspected fraud to the Police, in writing. As a minimum the police will have to consider your accusation.