Legal advice re: non-payment of child maintenance

if your case is with CMS then there is nothing a legal representative can do for you except relieve you of money.
because CMS calculates your entitlement slightly differently from the old CSA - they base there calculation on his gross income for the last tax year-they are in the position of not knowing his income.
I would assume that they are waiting for him to supply some income figures for his current employment, and hoping he will supply these; hence the 8-12 week scenario.
however, if he does not supply this then there is little that CMS can do. dealing with the self employed is the one major area where the CMS rules do not help the parent with care.
now for the reality; if he does not want to be cooperative then the CMS will have difficulty setting an assessment until, and if, he completes a tax return which at the earliest will be January 2019. the debt and enforcement teams will most likely not take action against him over the arrears as they cannot be sure of his financial circumstances and so the most likely thing that will happen is that the earliest you are likely to receive any money will be this time next year unless his circumstances change.
you should however try to keep abreast of what your ex is doing workwise and if you can find that he changes to a non self employed position make CMS aware asap