CSA wants me to repay them within two years

I have been paying off arrears as well as my normal payments with great difficulty for the last 2 years, my eldest of 2 children turned 19, so i foolishly thought my payments would drop. on phoning the CSA many times to get them to re-scheldule my payments, I was told I must pay ALL my arrears within 2 years and my payments were to rise to £800 per month.

I have never missed a payment and have regular contact with my 2 older boys.

My previous arrangement was manageable, but 2 days before xmas we were given notice to leave our rented property, we found another but rent increased by£225 a month, they wont take this into consideration..