There are several things you can do:
1. Go here:
http://www.hmrc.gov.uk/manuals/dmbmanual/dmbm595080.htm
It tells you that in certain circumstances the debt may be unenforceable under the statute of limitations. This does not mean the debt ceases to exist, but that it cannot be enforced by any Court action unless a County Court Judgement was obtained before the 6 year time limit expired. If you are as certain as you can be that no payment has been made on this debt in the last 6 years & that no-one (you or any joint claimant) has acknowledged to HMRC that you owe the money then you have a reasonable case to argue the debt is statute barred.
2. If that is the case, tell the debt collector in writing that you believe this to be the case & that you will not be making any payment. Ask for proof if they claim it is not statute barred. (Do everything in writing, & keep a copy & send recorded delivery.)
3. Note that if you still get tax credits, or start to do so again in the future the debt can be recovered from ongoing payments even if it is statute barred.
4. The debt will not have been sold on - HMRC don't do that, so you may have to contact them if the debt collector continues to chase you.
5. If you think you may have made payments or acknowledged the debt then you need to try to get the detailed evidence of how the amount was calculated. On the face of it, HMRC should give you this but it can be extraordinarily difficult to get them to do so. You may well want to do a Data Protection Act request to the Tax Credit Office & ask for all their records (paper & computer) & CD of all phone calls. However, I believe they destroy things after a set period so you may find they have nothing for 2006. In that case, & if you have no recollection of being informed of an overpayment it may be difficult (or impossible) for them to provide sufficient detail for you to know what this relates to & whether the amount is correct.
6. In that case you can complain to the Tax Credit Office & ultimately (if necessary) to the Independent Adjudicator & Parliamentary Ombudsman on the basis that you have been given no means of satisfying yourself that you do owe the money & natural justice has been denied to you.