On the face of it they are out of time to claim, because the time limit for claiming is six years from the date of the debt or the debt being 'acknowledged' by the debtor e.g if the debtor has paid some of it or interest on it or agreed they owed it . It doesn't matter that the alleged debt has been assigned ('sold') to 1st Credit.
Don't ,whatever you do offer, them any money or acknowledge that you owe anything now..
They can't do anything without proving the debt.That means going to court.Some creditors try issuing an invoice and then try issuing a notice in bankruptcy rather than using standard court proceedings of issuing a claim. They do that because it is more frightening to the innocent victim.The answer is the same: if anyone tries that ,you write back to them and the court saying you don't owe any money.Then if they want to go on they have to go through the court and prove the debt , that you owe money, and the debt is not too old, not out of time.They can't do any of that without giving you and the court details in advance of the hearing and you an opportunity to be heard.Then and only then can they get judgment.
These companies thrive on threats, not all of which are strictly accurate in law or fact, and the fear and ignorance of the public.Stand up to them.