Tax Credits Overpayment - but they don't want the money back?

Hiya, I wonder if this has happened to anybody else.
Basically, I got the letter from Inland Revenue last year (around March/April) to say I was overpaid by £153. They have me either a choice to pay in full in one off payment or pay by 12 installments. I couldn't afford a one off payment so I phoned them up and asked to be sent a booklet of 12 payment slips. I had recieved nothing. So I phoned them again last month (by that time I completely forgot about the debt as I heard absolutely nothing from them) and they basically said that my records showed my request but couldn't understand why I received nothing. They again promised to sent me the payment slips but more than 1 month on I still haven't received anything. What do I do now? Do I just forget about it as they don't seem to care or do I phone them AGAIN? Do you think it would be cheeky of me to say I won't pay them since they don't seem to want my money anyway? Your advice will be greatly appreciated.