Is informing the DWP tantamount to informing the Pension service?

My client had a huge PC overpayment (he was accused of not declared an occupational pension)  written off when he showed the investigators that he had informed the DWP incapacity benefit office at Makerfield that he was getting a occupational pension.

Although it wasn’t made clear during the interview why, when my client showed the investigator thast he had told the DWP about the occupational pension, the interview was abandonned.

The Pension Service have now declared a small proportion of the PC payments overpaid and are demanding them back.

Can anyone point me to case law or legislation which I can rely on the show that if he informed the DWP, he was effectively telling the Pension Service and therefore any payments they made subsequently are an official error?

Thanks

Dan