If the amount of child maintenance I agreed with my 'ex' in my divorce agreement is now longer enough am I just "stuck" with it because it was supposed to be a final settlement ?

Not necessarily! The Supreme Court has held that agreements made by parents about their child maintenance are not binding on them. Where there has been a substantial change of circumstances agreements on child maintenance can be opened up,even if the parent with custody undertook not to file for an increase. Children have rights of their own,independent of deals made by their parents about them,and these cannot be compromised. They can act,via the parental parent,as their natural guardian, and file for an increase in child maintenance from the non-custodial parent.