1. I think what you mean is that they are telling you you did not return the annual review form. If they do not get that (or a renewal done on the phone) by their deadline date, then they consider all the tax credit payments you have had in the tax year when you did not return the form as being an "overpayment" and demand them back.
2. In my view this is an appalling system because, as you say, you were entitled to the money but unfortunately the regulations that govern their procedures do allow for it.
3. Do go to the CAB for help. You (or they) should certainly ask the HMRC Data Protection people for a CD of all the phone calls, and a copy of all the documents sent to the Tax Credit Office. Meanwhile the people chasing you for the debt need to be informed there is a dispute.
4. There was a Parliamentary Ombudsman's report on Tax Credits late last year which included some criticism of the way they deal with this type of case. The CAB should be able to access it & it might help in making a case for this money not to be recovered from you.