I don't know if you are able to help, but last year I had a daughter with my new partner - and have had a number of issues with the CSA in getting my case re-assessed.
The assessment should have changed from the first notification of the birth on the 23rd June 2008 (a few days after the birth on the 18th June 2008 - I did call them before the birth, but they told me to call back after the birth!) - but instead was not applied until the 9th October.
The CSA "cancelled" the original re-assessment (without warning, and while I was obtaining all birth certificates, child benefit number, etc) - instead of the £600+ from the 23rd June 2008 that I calculated in overpayments, they have calculated they owe £150 since 9th October 2008 (I'm still waiting for that cheque - now nearly June 2009!). I overpaid £25.63 for 25 weeks (6 months).
I have been told that I cannot get these overpayments back, because the original claim was cancelled. Is this really the case, or the CSA just trying to pull a fast one?