They tell me I owe them £4,000, but can’t explain why

Hi, hoping you can help me,

I am self employed and a few years back CSA demanded I pay £60 per week to my ex partner for one child who lives with her, they came to this amount without having any records of my income for that year, they just plucked the amount out of the air. I obviously couldn’t afford it and asked umpteen times for them to send me a breakdown of how they had come to that calculation, they sent me nothing, they kept ringing demanding money even suggesting I borrow money from my girlfriend to pay it!

I continued to pay £5 per week as that is all I could afford based on the hours of work I had been doing and had been previously assessed at. I have been paying that ever since but my child is now 19 and out of full time education. I have had a call from CSA recently stating that I now owe them £4000 and again they could not explain to me how they had come to that calculation, I have requested again that they send me out a breakdown of how they have come to that calculation whether I receive anything or not this time I don’t know.

I have previously written to them a few year back asking to appeal against the calculation, again I hear nothing. What action can I take as I don’t believe I should owe that much money, can I get the debt written off if they won’t show me how they have calculated that amount? Please do not publish this story. thanks Mr Watson