Shared care yet ex gets money and wants more again

My husband has a son from a previous relationship and we share care with his mother. (182.5 nights a year to be precise!) 2 years ago my husbands ex partner decided to put a claim into the CSA! She gets the child benefit and tax credits and he son half the week yet she still wants more.

On Friday we received a liability order for arrears accrued by an assessment obtained through the national average for someone in my husbands profession in the area where we live despite having sent 7 copies of his tax returns, the first few they said they hadn’t received even though they cashed the cheques that were in the same envelopes! Then we eventually got letters acknowledging that they had received them and they still went ahead with the liability order. How can they do this, surely that is fraudulent and verging on a form of perjury?