The overpayments can be repaid on a sliding scale of either by A) reducing your payments by 25% or B) reducing your payments to nil payments until the overpayment is paid off. In order to have your payments reduced by 25% you have to meet the criteria of being paid MORE than the basic family rate of Tax Credit, so if you are not being paid more than that then your payments will be reduced to nil, until such time as the overpayment is repaid.
I know of someone who is in the unhappy situation of having to pay back the overpayment and due to the small amount this person is entitled to each year, as opposed to the amount they owe, they won't get any CTC for another 3 years by which time the child in question will be too old to be eligible for CTC so the family will be entitled to nothing anyway.
EXAMPLE - You are eligible to �500 per year in Tax Credits, but you owe them �2000, you will not get any Tax Credit money for 4 years.
I hope this clarifies the situation for you.