Hi I think anyone who has ever claimed workling tax credits or child tax credits have landed up with an overpayment!!!!
We also have an over payment of WTC even though we informed HMRC of a change in our circumstances and we have offered to pay back £30 per month, it is going to take us about 20 years to repay the money but if you really fight your corner and stand your ground, tell them you can not afford to pay back the money at the rate in which they are asking, complete an income and expenditure form and let them have that, if you telephone them make sure you have all of your out goings written down as they can ask you for this information.
Good luck