Hi, Pinky,
Basically, you don't have to do anything. They are in default of your CCA request and should not even be hassling for a debt that is disputed.
Unfortunately, hassle you they will, with the debt being passed around various dcas who threaten and bluster as if they actually know what they doing.
Have you received a letter from Incasso solicitors by the way? If so, they may be preparing to take this to court. If that happens, and they still haven't produced a valid, enforceable CCA, then they are on a hiding to nothing - and don't worry, you will get lots of help on how to defend a court claim on here.
BAE
PS - And NEVER phone them!!