unfair assessment by CSA

[AmyKay]

When my husband split from his ex, they made a full and final settlement regarding her and their daughter. He continued to make voluntary payments to 'help out' when she asked (very cunning would only accept cash and settlement was her idea and not in writing) . he could not have daughter unless payment was made in the end she wanted so much money (�500) he said he couldn't afford it so she brought in CSA.
After lengthy correspondence CSA eventually wrote to him saying they were satisfied no case to answer. (letter destroyed as no need to keep it or so he thought) just over 5 years later as bill arrived circ. �25,000 followed by Bailiffs. CSA will not listen to him only believing lies from ex. After means testing the 'debt' is �12,500. She has not informed CSA that child has been working for last 8 months. In order to stop more bailiffs arriving husband has offered to pay �10 a week and continue to fight his innocence, they wont accept this - offered �3000 borrowed from his mother as another full and final - wont accept this either. (Apparently asking the ex. if she would accept) We live in a council house, no savings no assets and soon no marriage at this rate.
Does the CSA ever listen to the man? PS they have just issued a liability order against him