If you did not receive an Administrative Penalty, and the date of the decision letter informing you of the overpayment was before the date of the bankruptcy order, the tax credit overpayment is a provable debt and should be included in the bankruptcy.
See paragraphs 40.44 - 40.46 of the Insolvency Service Technical Manual, this is what your OR will be referencing:
"40.44 Overpayment of state benefits
Overpayment of state benefit, such as housing benefits, jobseekers allowance or tax credits may be made as a result of mistakes, change of circumstances or fraud.
It has been held by the Supreme Court [note 23] that overpayments of benefits are contingent liabilities (see paragraph 40.9) and, as such, are provable debts. This decision over-ruled earlier decisions generally declaring such debts as non-provable [note 24] [note 25].
Such a debt incurred through fraud would not however be released upon discharge (see paragraph 40.178).
40.45 Repayment of overpayment of benefits where the overpayment was a provable debt
Bankrupts who have repaid, or are repaying, a debt which would be provable as a result of the over-ruling of earlier case law (see paragraph 40.44) may have a claim against the Department for Work and Pensions (‘DWP’) for the recovery of those monies. This is a matter between the bankrupt and the DWP. The official receiver has no claim on any monies repaid to the debtor.
40.46 Post-bankruptcy benefit overpayments
It is considered that overpayments which are made after the date of the bankruptcy order are a post bankruptcy liability, for which the bankrupt would be liable to repay, even if they arose following an error (the decision to overpay) which occurred before the order."
Hopefully this 'administrative error' will be resolved soon.