I wanted to share my story of disputing an overpayment in case there is anyone else that was in the same situation.
For my 2008/2009 renewal my details were taken incorrectly and they completely missed off dh's salary so took my small salary along with the childcare as the basis of my claim of course this gave me huge payments - nearly £300. In the space of 2 weeks tax credits paid me over £900. I rang and queried it and they corrected the mistake but this left me with an overpayment.
I left it and accepted that I was going to have to pay it back until I read something and contacted TC about the overpayment and that I wanted to dispute it as my details were recorded incorrectly. I contacted them and completed the form and returned it. after a couple of weeks I had a letter saying that the overpayment was nothing to do with the £900 they sent me
The letter stated that it was due to payments when they corrected the details would give me an overpayment and I only told them to stop the payments when it dropped to less than £10 when my childcare stopped. All of this was within about 2 months so wouldn't have produced a large overpayment. I rang the TC again once I received the letter and explained again why I felt the overpayment wasn't due to my error and the person I spoke to agreed and said I should put these points in a letter and ask for the case to be reviewed.
Today I have had the overpayment refunded and tax credits resumed as they have admitted they made the error.
So if you have disputed a claim and you really think it is their fault but they won't admit it persue it as it is worth it