I have complained to CSA Birkenhead

This is the complaint I have just sent the CSA in Birkenhead:

I have informed the CSA a number of times recently about a change in circumstances i.e. my son turning 19 & leaving full time education and now living in Derby. this should have resulted in a reduction in the amount I have to pay each month. Although the CSA aknowledge that they are aware of the change, I have still had the original amount taken from my bank account.

I had not accounted for this happening this month. I would like to make an official complaint about this happening and I would like to know when the issue will be rectified and what compensation I will receive i.e. loss of interest.