They do have this right , I had something similar re a SW over payment, Ironically in the same year. It was not my fault and they clearly accepted it was an error on their behalf . They facilitated a repayment schedule . I don't believe there's any statue of limitations re Revenue or SW liabilities.
Illness benefit a little more complicated on how its determined (payment) but is I believe calculated on previous earnings so how you check or query this , I don't know, perhaps other might have advice on this .