CSA took a large sum of money without warning now we face losing our home

My husband and I have been married for 8 years, together for 15 and have 3 children together. He had an affair and consequently has an illegitimate child who is nearly 4. The woman he was seeing was blackmailing him for the first couple of years for money ( I didn’t know about the child until 18 months ago) and when I found out, I (stupidly) told him to go through csa.

His payments are worked out at over £200 a month- his wages support our family of 5 and I’m a part time worker as I’m a full time student. Knowing we couldn’t afford this money, in June last year, I contacted her and asked her to consider £150 a month instead. She said she will think about it. We never heard another thing so started paying £150 instead of the £130 we were previously paying. Next thing we hear, this month, my husband has had over £400 taken from his wage without any prior notice! He contacted csa and they said its for arrears that have built up as we haven’t been paying. They also said they wrote to us in December. We never received this. We are now facing losing our house and all because of a stupid mistake.