CSA say there is no more money from my ex

My children were abandoned by their father in 1999. By 2001 divorce was finalised and judge recommended getting CSA involved because ex threatened to stop working. He did.

Throughout the last 9 years I have had to contact them with any information I could so he would be made to support his 3 children. However, he has worked cash in hand (CSA would not pursue), his new wife told them he did not live at address (CSA would not follow up).

Eventually, he bragged about his new company, new premises and how well they were doing in the press (CSA say he doesn’t have enough income to pay anything)… My youngest son is due to leave college in 3 weeks time and I am now told there is no more time and they cannot be retrospective in assessments.