anyone every managed to claim/get money back from the useless wankers??

i've been trying to get my payment amended since may as my partner and her daughter moved in with me so i should pay 5% less maintance,now i dont mind paying the money there my kids and i've always provided for them but the csa take the piss taking the extra each fortnight out my wages then take an extra £234 out and said they'll amend future payments but still nowt happened.i've worked out they owe me nearly £400 quid is there any way i can claim this back- court ect?