CSA refunded me after I forced them to review my case

I was making payments to the CSA for £ 315 for 1 child from 1998 till 2004/5. In 2004/5 after intervention from my MP I managed to force the CSA to review my case at which point they refunded me over payments of £3500 and reduced my payments to £58 a month. I continued to make monthly payments until I contacted the CSA in Feb 2011 when the child I had been paying for turned 19 years old. I asked them if my payments would stop and was informed that a final review of my case would take place and the case should be closed. 2 months later I received notification through my employer (I am in the forces) that the CSA would now be collecting £ 315 a month. As you can imagine I was some what confused to hear this and contacted the CSA.

I was informed by an operator that I had an outstanding dept of £20,000. When I enquired where this dept had come from it was explained to me that there was no evidence of me making payments prior to 2004. I explained that I had had a review around that period and had in fact received money back from the CSA. The operator informed me that I needed to produce evidence that I had made payments prior to 2004 which I did by producing pay statements covering the period of alleged non payment. I also enclosed all paperwork showing the outcome of the review in 2004/5. Form Apr 2011 till Sept 2011 I contacted the CSA monthly to try and sort this out and eventually managed to stop them taking money out of my wage whilst they investigated the dept.

I deployed to Afgan in September and was informed by CSA that by my return in Jan 2012 this matter should be resolved. I contacted the CSA in the first week in Jan 2012 as I had received no correspondence from them. I was informed that I still owe £20,000 but the operator could not explain why only that it was not due to them having no evidence of me making payments before 2004.

I have now been in weekly contact with the CSA asking for a written explanation as to why I still have this dept and even thought I have been promised every time that I will get a letter ‘in the next 5 to 7 working days’ explaining the circumstance of this dept, I have received nothing. I’m not sure where I go from here, I am tried of hearing the same thing ever time I call, non of the operators can tell me what is happening and I have asked for anyone at all to call to explain or for a written response but have received nothing. I would appreciate advice on what to do next?