Applying for backdated NI credits

Have any other advisers ever dealt with this?  It is totally

One of my client wants to apply for backdated NI credits for period from Sept 14 until Feb 15.
According to CPAG guide p 990, it should still be possible to do this, by applying to Jobcentre Plus.
But what department should the client address his letter to, so it does not end up in the usual JCP black hole?  He enquired over the phone and was sent an ESA1.
Client was claiming JSA in March and then working from May. He is currently making a new claim for ESA anyway. Maybe he can send the backdating request with the new form?