No you can agree a figure between you, unless you as the parent with care of the children receives certain means-tested benefits, in which case you are obliged by the DSS to apply for an assessment by the Child Support Agency. If that you refuse to make an application, you may be penalised by having your benefit payments reduced by a draconian amount - 40% - for an unlimited time. Other single parents who do not receive benefits may apply to the CSA for an assessment if they wish to, as long as there is no Court Order in force regulating payments. They may however prefer to agree a figure for child maintenance, bearing the Child Support Act calculation in mind.