NIC Office and the bill that time forgot

Preparing the 2009/10 Tax Return it was clear that deferment of Class 2/4 NIC was appropriate. We duly submitted the form for 2010/11 and this has been returned by the NIC Office. They have refused deferment on the grounds that £300 is outstanding from a previous deferment in 1999/2000.

This is the first that we have heard about this and the client has no recollection of it. He was dealt with by another firm at that time but there seems little point in asking them whether they have records from 10 years ago.

What would you do? If the NIC Office is incompetent enough not to pursue arrears for 10 years they are presumably incompetent enough to have received the payment and not noted the record correctly.