Go here:
http://www.hmrc.gov.uk/manuals/ntcmanual/payme nt_opayments/ntc0780052.htm
Read it, & also the next page (click for that at the bottom of first page). It makes it clear that they write off (remit in their terminology) any outstanding amount as at the date of discharge from bankruptcy. If you were still getting tax credits while bankrupt (I don't think - from your post - that you were) then they will have been collecting from the ongoing payments until discharge.
You may well be dealing with their Debt Management arm (? at Cumbernauld). It is possible that they are not aware of the procedure set out in the manual so write to them with details of it, & also a copy of your bankruptcy order and discharge certificate. Send by Recorded Delivery & keep a copy of the letter. If they refuse to write it off go through the HMRC complaints procedure (they have a booklet on that).
Incidentally, if the tax credit claim was a joint one with someone who is not bankrupt, then they can continue to collect from that person.