Backdating of SDP on GC

My client was awarded AA 4 years ago and should have been getting SDP added to her GC but isn’t.  When I queried this with the PS, apparently 2 PC10 forms were sent out in 2006 for the SDP, to confirm that no-one was getting CA to look after her (PS knew she lived alone and was getting AA) but she didn’t remember getting or completing them.  I have ordered another PC10.  Is there any chance of getting the SDP fully or partially backdated and how best can I request this?

I had a very similar case and was successful in getting the SDP backdated 7 years. I included evidence from client that no CA claimed etc, lived alone throughout and requested full backdating. It was even processed quite quickly.