Incapacty Benefit Reduction due to Pension

Please help, I have received a notice of incapacity Benefit Reduction due to Pension.
My IB was being reduced by £32 a week now it will be reduced by £43 a week.
My concern is they say I owe them £980 since April 09. I do have copies of the about your income form & payslip that I sent in 04/09, also a I have copies of the IB6 with the earlier form stamped by the jobcentre & payslip.
I also sent a form this year, and they returned the payslip.
Should I have to pay what they say is an overpayment, when it is clearly their mistake.

If you supplied them with all relevant information as soon as you should have done, then they can only ask for a repayment of the overpayment, not insist upon it. This is in contrast to the position where the overpayment arises out of a failure on your point to supply all necessary information promptly.

If you have difficulties with this, it would be worth seeing a welfare adviser or contacting your MP.