CSA payments ( or what ever it is called now) are payable till 21 years or age if children are kept in full time education with no breaks and no paid jobs. This is also payable for that period of full time education ie) if the child is 29 and the period the biological parent was absent is age 12 - 21 - then they still are liable to pay this period even though the child is now and adult under historic debt law .

Ring CSA - and say to them that the biological parent isn't paying and the CSA need to get the debt collector to pay it. You will lose a bit to a fee but he will have to more than you to the debt collector and the CSA and keep the children in full time education no saturday jobs till they are 21 and he will have to pay it, but you get your tax credits and family allowance and ema £1200 a year till 18 and Welsh Grant £1500 till they are 21