Csa issue on payments long story?

Hi I'm new on here and hope some of you can give advice before I phone csa tomorrow, I have 3 children with my ex husband aged 14-10-5 in February 2014 he lost his job he was paying £200 per month not through csa though but an agreement between us both on losing his job this ceased, then in sept 2014 he moved 300 miles away,h only speaks to the 10 year old by phone, my 14year old wants nothing to do with him and my five year old has nothing to do with him this is not down to me either in November I went to csa and started a case with them I know where he lives etc, however he told my 10 year old he wasn't working however I know differant and traced where he works and told the csa, I text my ex and informed him on this he said he would pay £200 per month again, I think this woke him as I know he had no intention of telling me,however next day the csa sent a letter stating he should pay £396 per month starting 1Feb 2015,that's the last I heard until today when a text came from my ex saying the csa have sent a letter saying he only has to pay £7 per week until dec 2015 when it will be reviewed also a happy new year which has really riled me up, on looking on my case online today this is true what he has said, it looks like they have just worked from February until dec when he wasn't working, my point is I have three children to raise which I struggle with as one has disabilities, I buy all there clothes food etc and now for my ex to be earning around £1500 per month to pay £7 per week disgusts me, he will be laughing at me as he is not right in the head, my gripe is surely why can't the csa contact his work surely he will have a contract on his salary etc and work from this,why does it have to work from him being on benefits, surely in December knowing my ex when a review will for up he will leave his job so again I will get nothing and this circle of working and not will go on and advice as I felt so low today as I thought finally I would get help with my kids but now I feel this will never happen thankyou for reading and sorry it's so long x