CMS on my case as ex denying payment even though I can prove it

I started paying CM to my ex at a rate of £25 a week, I am on a very low income, despite not having to.  Then I received a letter from the CMS saying I needed to pay £65 a week. Then they revised it to £19 a week. And the whole time I was paying £25 a week.  Next I get hit for non payment, despite still paying then I get threatened with court.  So I contacted them and presented evidence to show I had been paying it and that my ex had been spending it.  I have now been told because my ex has denied I have paid it, because it went into a joint account and even though I have not spent a penny from that account I am liable to pay the whole amount again.  I have paid just under a thousand pounds and they want another thousand.  I don’t know what to do I can’t afford to live now.  Please help.