Advice needed

Non resident parent 19 year old daughter paid while working under the old csa. Several periods of unemployment however continued to pay out savings and never claimed benefits. Recent notice for over 4 thousand pounds and doe put in place from child maintenence service who will not tell me what period and how this amount has occurred. Previous letter years ago saying child's mum had asked for money not to be collected. I'm getting no where on the fone all the cms say is they don't have access to the old system. How can I prove I don't owe this money when they won't tell me what period it's for how it's worked out and what they will accept as proof.