Arrears have built up because of the CSA

Hi, My CSA payments come directly from my salary & have done for the last 10 years or so , that benefits me as being a merchant seaman & away from home for months at a time i am happy in the knowledge that my payments are never missed , my problem is this , i received a letter from the CSA saying that i was being reassessed , fine by me , so i sent in all the documents they asked for , wage slips etc , i then received another letter from them saying i had been reassessed & my payments where to double , again i had no problem with this , then they said i was in arrears , how come i asked , your new payments are being backdated to march they said ( i received the reassessment letter in august ) that is how we operate , how can they do this , when i complained through my MP they sent me a cheque for £20 as a token gesture , a token gesture would have been to wipe off the arrears which i have through no fault of my own , all this happened almost 2 years ago & my arrears are almost paid off or are they , will they hit me with something else that is hidden.