Tax credits have overpaid me by a huge amount too, about 2-3k which is also their fault! I rang them at end of Oct to change my claim to a joint one and they wouldn't let me do it, rang numerous times to try and change it but they didn't stop my single claim, just carried on paying it til March!! Now I'm goong to lose out whilst I have to pay it back. They are seriously unreal. But you can appeal the overpayments if you think its their fault, so I'm going to try that! Xx