Here is the website for CSA www.csa.gov.uk. For new claims you need to click on the right hand side of the page.
CSA definately DO NOT keep any of the money - for one child you will get 15% of your ex's wages. It goes up to 25% for 3 or mor children, this is the maximum. There are some extra rules ie. if he now lives with someone and she has kids (even if kids are not his) he gets a percentage taken off what he has to pay (though it is not that much). Also if your kid/s stay overnight with him at least once a week, then he can reduce payments by one seventh, more if they stay more than one night a week.
The men folk sh8t themselves at the mention of the CSA - if you know how much ex earns, phone CSA and ask them to work our calculation for you. Basically it is his weekly TAKE home pay, well you would get 15% of it for one child.
If you are not happy with what you are getting and CSA are getting you nowhere I would make an appointment at the Citizens Advice Bureau, I have always found them really helpful with all family matters.
LISTMAKER - ok, i am in similar situation to you except my ex is italian and i know who he works for. As you say if they are overseas CSA can't do anything BUT you should phone your local magistrates court and ask them about a REMO - www.csa.gov.uk/newcsaweb/faq/application/abroad.asp - hope this link works, have a read - at the end of the page is a further link. Basically the Home office may be able to get local courts of country he is in to make him pay. I am going through this process now so will update on here later to let you know about it.
Good luck to all, and make them pay!!!!!
Sorry this is such a long message, I get on my high horse with selfish men