Firstly, calm down. There's no rush re decree nisi.
Secondly, go to the CSA over child maintenance. The court won't be able to help you here. Ring them today, so that the maintenance will be backdated to the date when they contact ex.
Thirdly, depending on the circumstances in which the agreement was negotiated, you may be able to put it in front of a judge, but that might be in contested financial proceedings. You'll need to take proper advice from a solicitor over that. Look on the resolution.org.uk website for details of a solicitor near you, or on the law society "find a solicitor" website for a member of either the family law panel or advanced family law panel near you.