If you don’t agree with the Helpline's explanation of the reason for the overpayment, you can ask HMRC to look at their decision again. This is called disputing an overpayment. You can do this in writing or by filling in form TC846 which you can get by calling HMRC on 0345 300 3900. You may find it helpful to attach a separate letter to the form. You should give all the information you can, including exactly what you did and why you think you should not repay the tax credits. If you have any evidence of a mistake made by HMRC, for example, copies of any correspondence.
