At my wits end - Advice please

my ex dragged me through court to try and get full residency of our 2 children, but the courts ignored him and awarded the children of 13 and 11 a 60/40 shared residency, he then appealed and that was also thrown out, all the through the court process he was getting legal aid which dated back to 2013 and it seemed he could get it whenever he needed it. I had to use my savings to go through the court process.
In December of 2017 he dumped the kids at mine and said i cant handle them, the kids have been with me since, he texts them every couple of weeks when he feels like it, but they dont respond and they dont see him at all.
He works full time as a nurse and is refusing to pay any maintenance, so CSA are dragging their heels going through the process of finding out his earnings etc, after 6 months they have finally found out where he works and how much he earns, but he has told them he cares for them 3 nights a week as per the court order, but i have explained to CSA that is not true as the children have not stayed with him for 8 months, but they said they cant do anything and that i need to get the court order changed. but firstly i dont want to drag the kids through all that rubbish again with social workers or CAFCASS etc, plus the judge put in the court order that no further applications etc could be made for the next 5 years. CSA have said there is nothing they can do about it, even if i do prove to them that the kids are with me all the time.
So he can lie to the CSA and nothing is going to be dont about it.

Is there anyway i could go to the court to get a lump sum payment from him via the courts as he is ignoring me about the forthcoming school uniforms etc and other costs and things the children need.

Thanks in advance for any advice