We got overpaid for a year of tax credits, around £3000 - my fault as felt would be in a better position to pay back than loose £200 pcm last year, which we are.
I know there have been a few others posting that are in a similar position with owing large amounts so i just wanted to let you know about my relatively easy experience of setting up repayment.
I read on-line that if your offer of payment means you will pay of the debt in less than 10 years they will not ask for income/expenditure details. The direct number to call to make an offer of payment is 0300 2003804, i hardly had to wait, told the man how much we owed and that i could pay £30 per month, he checked (i assume that it would be paid off in less than 10 years) and said it was fine and set up a dd to start on 1st Sept. All in all very painless (at the moment!) So if you're in the same position, give them a call, make sure your offer of payment is enough to repay in 10 years and you should be ok.