Desperately need some advice, our son recently had sdp added to his ESA after we became aware that he became eligible back in 2018. We informed DWP about change if address and that he was living alone. The lady asked was anyone getting carers for him. No as I already get carers for my disabled daughter who lives at home but I would still be doing some unpaid care for him. No mention of SDP or an is10 form. Fast forward and after numerous people saying he would be eligible for a backpayment as he was and still in on pip higher rate with care and mobility, ESA support group, living alone and nobody getting carers or carers credit. DWP sent a letter today that says they won't backdate it as "why did you not apply in the last 7 years". How can you ask for something that you know nothing about. I had never heard of an Is10 until 4 weeks ago. How do I move forward in doing a mandatory reconsideration as I am his appointtee. I have his original tenancy agreement How to I fight this and is it worth fighting.