I sent these 'nice people' letters promising payment of the alleged debt if they could provide proof of contract with them and proof of debt by providing the full accounting for the loan. Under English Common Law one cannot sell a contract to another party because they will not have signed it (almost no company signs contracts these days though so that alone is enough to stop them. There is also the Common Law principle of "Unjust Enrichment". This is where you have the Common Law right to demand the full accounting of the account so you can look into it to make sure they are not unjustly enriching themselves at your expense. Companies like Cabot buy alleged 'debts' for pennies on the pound.
Stop using statute law with them unless you want to use the A4V or promissory note idea found on this site and work on just English Common Law (or just Common Law if you live outside of England and Wales). As long as you promise them payment of the alleged debt if they can prove the lawfulness of their alleged contract and that a debt exists and you actually owe them money by providing the full accounting you'll be fine because unless you contract with Cabot you don't owe them anything. In the first letter I sent them (after starting to exercise my Common Law rights) I put them under Estoppel and told them this meant they could not lawfully take any action against me unless they were able to provide the proof of their claim.
When they came after me I paid them £1/month for just over 6 years because I didn't know my rights until I had had enough of their harassment. I told them in my letters and fee schedule to them that if they failed to provide the proof of debt and proof of contract to me they would owe me damages for the treatment they had given me over the past 6+ years. They've not provided any such proof and whenever I wrote to them they would just write back that they were investigating and would contact me again. They've now confirmed to me (over the phone but not in writing) they won't ask me for payment again. Could it be something to do with the fact I told them that every time they asked for payment of a debt they had not proven they were committing fraud (using the definition I gave them) and every time they threatened action if I failed to pay a debt they had not proven existed they were committing extortion they would owe me £50,000? I wonder
All the best.
Nothing I say here can be taken as advice of what you should do because you need to do your own research though I and others here can help you with that.
Think like a man of action, act like a man of thought.