In UK what do i need to do/get legal custody of my children? I have them in my care through our divorce?

I am wondering what do i need to do to make it a legal thing, my ex husband has them once a fortnight agreed on by our divorce through our solicitors. I have parental responsiblilty he has tried to get custody but his solicitor said he wouldnt stand a chance so he stood that down. What can i do to make it official? I shall make an appointment with my solicitor in the meantime any advice would be great he does pay maitenance but it has took him over 18 months to do it. I am in the process of getting the backdated from the CSA as i am very vindictive at the moment with him!!