My ex husband is playing nasty with csa?

he never bothered with his 3 girls until i remarried this aug i moved in with fiancee in march, he hashis girls for 2 nights a fortnight and a lot of the time they dont actually stay as his as they stay at there bfs house, so technically when he collects them on a frid fortnightly only 1 stays at his house. he is now claiming back pay from csa why now after 7 months? and also he had my eldest daughter stay at his in mar until she finished her exams she came here at weekends i have them al fulltime part from his occasional access he has told csa he had eldest fulltime 7 days a week for 6 months! does he have a right to claim? im awaiting a seroius op andcannot work i need the csa money i have to fill in the form and sendit backwith 7 days. Whos right?