HB
I'm afraid there is no definitive set of criteria for the Severe Disability Premium, so we are unable to say whether you meet the requirements or not, but in general you need to show that you and the other people in the flat lead seperate lives. Things that will be considered are seperate rent books, seperate bedrooms, you may need to show how you manage communial areas such as the kitchen.
I'm afraid the only advice I can give is for you to keep chasing the DWP.
Good luck
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems