Anyone know the answer to this question?

Single parent negotiating maintenance with now estranged father of her children. Aware that any paid will be disregarded in full for CTC purposes. However, how will her Income Support be effected, if at all remembering the maintenance is for the children. I know I should propably know the answer to this but I don't.

It is deducted pound for pound (minus a small disregard in a small number of cases).
I have to write a letter to one of my clients to break this to them.
It is rather counter intuitive,but I guess I should stop expecting the benefit rules to make sense.

Also - anyone claiming IS has no choice but to go through CSA, so negotiations won't be able to lead to any long term agreement.