B****y CSA Nazi's

Unbelievably, the CSA have decided after 4 years with absolutely no contact whatsoever, I owe a staggering £8,000 arreaers, what's more, they've made an order against me to the tune of £120 per week. I received no notice, no imformation, no letter, nothing, just strolled into work one morning, opened my payslip, and there it was !!
I contacted them to explain that:
A) My ex partner and I had reached a verbal agreement whereas I took sole responsibilty for ALL our joint loans and finance in lieu of child support payments, which were compounded and verified when I went bankrupt and debtors pursued my ex, who explained the situation to avoid her liabilty to those debts succesfully. Part of the agreement was regular access to my children which was revoked by her as soon as she found out I was seeing my now (wonderfully supportive) wife. The age old question of " why should I pay for children YOU won't let me see" ? arose. On the one occasion I insisted on seeing my children, she promptly called the police who arrested me and formally instructed me to stay clear, which was later backed up by soliocitors letters and warnings.
B) I explained to them (with difficulty) that the amount they were persuing was a total mystery to me as I had had ABSOLUTELY no contact from them whatsoever, and therefore failed to understand how I could be in arrears on an "account" I had no knowledge of. Seems the staff at CSA Belfast have difficulty understanding the concept of fathers being honest and trying to remedy these difficult situations. Their response was unco-operative, unhelpful, at times rude and on one occasion offensive. Are they employed to make personal comments and judgements ? I also explained that she was claiming for a 6 month period that we were still together in. All this fell on administrative deaf ears.
C) The amount they were deducting left me unable to support my family and meet weekly financial commitments, meaning that the children and wife that I DO support, missed out and suffered because of a spiteful ex partners vengeful nature, not only that, but eventually it led to me losing my job and slipping into severe depression. Thankfully, my wife was very supportive and understanding, but it doesn't come close to aiding the feeling of desperation, humiliation of not being able to provide for a loving family and subsequent breakdown that we're all still struggling to come to terms with.

Anyone any idea's what I can do to sort this bloody mess out and get our live's back on track ?