Sdp

hi just asking for more help iv recently been accepted for sdp from 31/1”/18 but I should have been awarded it from April 2014 but dwp are saying I’m not getting backdated money bec I never told them my son had moved out in April 2014 because I didn’t genuinely think I had to because he had reported the change bec he moved house and got housing benifit for his new property dwp have send me a letter saying no backdated money will be paid so I filled in a mantory reconstdiration form asking them to look at this again I also provided a tendency agreement from my sons old property and letters to prove I got single occupancy on both exactly the same dates when my sons tencany started but dwp are still saying I should have informed them off him moving out , but because he was a non dependent I didn’t think I had to as he had already done this on his own behalf can anyone tell me what to do next I’m lost they owe me approx £12.000 and I feel robbed and angry about this as this was a genuine mistake I told my council and that’s all I thought I had to do