If the money was paid, and the decision that you had to repay it was made, before you went bankrupt then this is a debt in the bankruptcy.
The only question here is when the decision you must repay was made. As the money was paid to you in 04-05 that decision should have been made sometime in 2005 & you should have been notified then.
I assume the letter you have came from the debt recovery outfit at Cumbernauld. If so, the decision must have been made some time before they wrote, because the decisions are made by the Tax Credit Office & (normally after writing to the claimant) then referred to Cumbernauld for action.
I suggest you send a letter to them with a copy of your bankruptcy order & tell them you understand this is a bankruptcy debt and that you cannot legally make any payments on it. Also, send a copy of their letter & of your reply to the Official Receiver or Trustee who is dealing with your case. Letters by recorded delivery & keep a copy.