You have to go through the proper channels before you go to the independent commissioner. And the MPs havent got a clue.
First of all this sounds like an interim payment.
This is what you do. It sounds that you are speaking to the wrong department. Put a complaint in writing to the complaints department of the CSA send this by either a fax or recorded delivery this is very important as you will need confirmation of everything you will be saying to them. They have to respond to you within 15 days. Ask for a full breakdown of your maintance assessment. This should entail the maintance needed for the PWC your exempt income and your protective income make sure that the corresspondence they send you clearly states this info as CSA are good at sending you just the break down of the calculations. Tell them it is the full break down of you maintance assessement that you want with the info on it that i have just mentioned
Write to the data protection department and ask them for all the paperwork that they hold for you from day one. Send a copy of this letter to the complaints department who can pass this on to the revelant department. They have to reply to you within 40 days. Be prepared for some reading but do read this.
They have to by law send you corresspondece of how they have worked out your maintenance before any deductions. You say that they have taken 550 out off your wages they cannot do this without an order called a Deduction of earnings order. This has to be obtained through the courts. You would have received confirmation of this. You say your partner didnt get any word of this. Are you still in good talking terms with her. If so. She can stop this by just contacting CSA to cancel this. She does not have to use them anymore. She can have a private agreement with you with out DHSS knowing if she doesnt want them to know. But remember if you give her any money please have a paper trail. And get her to sign a bit of paper stating that she wanted a private arrangement, or this can come back and bite you when you have an argument in the future.
I need to know everything of your case to help you. They just cant take this amount of money without informing you.
Good Luck