Severe disability premium

Hi, I am after some advice. Our son who does not live with us claims ESA old style and PIP enhanced for both elements. He also gets housing benefit but nobody claims carers allowance for him. I have done a benefit calculation as I know he will be getting a migration notice at some point. However the calculation said he should be eligible to claim Severe Disability Premium. Is this something we should attempt to do before his migration notice comes through. Also I am his appointee so do I just ring up and ask to apply to have this added on and should it be backdated.

Thanks David, yes I knew it was me as the appointee who would have to call. I rang first thing this morning and fortunately spoke someone lovely. She said she would complete the form for me and asked some questions. She said that his payment between now and the next 4 weeks would increase. When she looked back to his claim he originally lived at home but in 2018 he moved into different accommodation. She could see we had informed them of a change of address but that the person who had taken the call had possibly put down that I was at the same address. She was going to send it to someone as he could possibly be able to have it backdated to March 2018. So just a wait and see. I am thinking this will move his migration from ESA to UC sooner.