SDP Back-date

My client has received DLA MRC for several years, but wasn’t entitled to the SDP premium on her IS because, although she lived alone, her mother received CA for looking after her.
However, in April 2009 her mother died, and although the DWP sent her an IS10, she ignored it, so she never received the SDP.
She came to see me a few months ago about her DLA - and the SDP is now in payment. She received a 3 month back-date.
Is there any entitlement to a back-date to when the mother died in 2009? I think not - but would just like an expert opinion. Many thanks.

Thanks Tony. But I wondered if the application should have been made within 13 months of the date of change? Reg 8 (3)(b).

Will do. Thanks.