Pip Award

My husband finally had his PIP award arrive on saturday and thankfully they have granted him enhanced on both elements for an ongoing period. His renewal form was originally sent in december last year and even though a hiccup with the assessor in march and his original award being stopped temporarily for a month then it being given back to him, the assessment was carried out paper based and the decision has just been made. My query however is that they are stating they will only back pay until the date of the letter, 24th sept, is this correct? Surely it should date back to when they recieved the renewal forms and evidence?