Back dated CSA

if there is still an open case then CMS should still try to pursue any arrears that are still due. it will be up to your father to pursue this, not you. you could get your father to let you become his representative -you need him to write in- and then you could deal with CMS on his behalf.
first thing is to establish if the case is still open, and then how much the arrears are.
the problem is that CMS may not be able to find her (because she remarried, moved abroad etc) and unless you can offer them some information like a new name or address they will not undertake more than a cursory check for her as they are not an investigating agency.
assuming there is still an open case, and that CMS are able to find her, they still have to find her money, so if she is a housewife with no income of her own you still may be unable to get any money.
because of the age of the debt some, or all, of it may be legally unenforceable adding further problems.

so, check there is still a case, and if not there is nothing further you can do.