SDP and hinchy? Backdate??

Client lives with disabled mum. She gets SRP and he gets SDA. Both on MRC DLA

He gets top up of IS for incapacity, inc Carers premium.

His mum claimed CA for looking after him (therefore he couldnt have SDP), but CA stopped 4 years ago when SRP kicked in. Underlying entitlement in place now in her PC award.

He has been entitled to an SDP when the CA stopped being paid, but obviously they never knew that.

He has serious learning difficulties, and his mum has bipolar.

Im going to get his SDP added, but im wondering about backdate of SDP with regard to “Duty to inform of change of circs” etc

Thoughts as to my chances??