sorry if it's already been said, but if you go on the website, you can work out what the minimum you should be paying based on your yearly income.

http://www.csa.gov.uk/new/calculate/

My girlfriend has contacted them about her X not paying his way for her boy and they've done absolutely fook all, useless c*nts.

hope you've kept a record of EVERY payment made, this may help the CSA figure out that you have actually been paying as if its jus cash in her hand she could easily say you've paid nothing
HTH