[Solved] appeal on pay calculation

Nov 16 I informed Child Maintenance Dept I was leaving my job and joining a business as self employed which my partner was buying. Started business Jan 2017 and told CMD and gave new address. Told them income would be significantly reduced but we wouldn't know by how much till end of a full financial year April 2018. Two months later ex wife issued new claim, calculation was done on former job earnings. Rang numerous times to tell department income would not be as much as former job..they said I still had to pay although in a couple of calls I was told to ignore arrears letters as sent out automatically and just get financial info to them when available. April 18 my HMRC info showed I paid nothing in for tax purposes after leaving job Nov 16..so clear wasn't earning. CMD still insisted on payments and I asked for mandatory reconsideration..it was rejected on grounds I didn't get financial information to them on time. I have appealed but CMD say have to pay all arrears, some £2,600, and whatever happens in appeal this amount will not change or be reduced ? They say case is closed, payments cant be reconsidered now and basically its tough luck. My actual earnings for period in question were one third of what they were when I had an employed job. It seems the decision is final even before an appeal takes place. I was informing them all along of the situation but they say I didn't get my earnings to them within a 30 day deadline..only way I could have done that was to pluck an imaginary figure out of the air. Anyone have this sort of experience, any luck with tribunals, or any other advice.