dont worry about it. A debt collector is somebody who works for the company. They have no authority and no legal status. They have as much right to enter your home as I do. You dont even have to answer the door to them.
If you look on the net you can find some letters to download called "Statute Barred", you can print them off and post them (recorded delivery) to the debt collectors, basically it is just saying that the debt is statute barred and they can go away!!
If there is a CCJ in place it is up to them to prove there is but by the sounds of it there isnt so unless they can prove that there is one dont worry about it. There is nothing they can do and they cannot enforce the debt.