Hang in there. They will have made the assessment based on what she has told them and they may have checked your tax returns with CMS (Child Maintenance Service). She will have told them you have no nights with the child so it will be the maximum assessment. There isn't a CSA any more.
Have you tried using the online Child Maintenance calculator to check if their calculation is correct? How have they said it is to be paid? Directly or are they taking it from your account and charging interest for doing that? When you've checked the child maintenance calculator you can then call them to discuss it - depending on what the calculator says. But check on here first. If it's a correct calculation you'll probably have to pay it until you get a Child Arrangements order when it will reduce depending on the number of nights your child is with you. You could tell them it was 1 night a week but she has suspended that and it's currently going through court to get time back. But check this first and post back. Unfortunately they don't take things like mortgage payments or bills into account. Some people have had to change a car or get a lodger in eg. It could be an idea to ask for a paternity test as part of your court order.
Use this calculator to work out an amount of child maintenance for your children.
www.gov.uk