Murrysmum - When DH & I seperated his CSA was based on the previous financial year - over 50k. I rang them & told them that he wasn't going to earn that much this year. I think he signed a form (or it may have been a phone call) & they adjusted it.
If he earns more than he tells them, then he'll owe when tax is done next.
CSA sets the rate & she can't decide weather they change it to suit his income or not. Thats up to them. If she says no, I think its probably because she is sticking with the CSA arrangements so every thing works out even.