The CSA say I owe a massive back payment from when I was ill – what do i do?

3 and a half years back, my then partner walked out on me taking my son. I’ve been pursuing her through solicitors to try and see him but still nothing after all this time. This whole time I’ve been paying csa every month, two years ago I had surgery that put me out of work for several months.

Now csa are claiming that I owe them a massive back payment and that they are taking almost £300 from my £1000 monthly wages. I can barely live on the money left over. I have tried contacting them and have had pretty rude responses. No idea what to do now