CSA taking too much money

My case started in 2009 my who is now 13 lived with me for the first 9 years as my ex had a good old time then wanted him back and did it a very bad way. 2009 i was working full time the csa told me i had to pay £56 a week as which time i had to go partime as work was drying up i told the csa still i had to pay £48 a week but was only giving £120 a week so they could not take any as my earring was not high.

I have three young kids to they used there money as my earring to the arrears just kept going up.

My hours was up and down for this till april 2011 never once did they change anything so my arrears over £4000. i was sacked from my place of work because of the csa.

I am now back at full time and want to pay but they are taking over £60 a week off me and it is killing me it is the arrears that sure never amounted to so much. They had me as getting over £400 a week over 2010 i told them never once did they change it.

Thank you if any body could help.