Can partners ex claim CSA now mutual agreement ends?

My partner has been paying a agreed amount of money to his ex wife,that was arranged by themselves for the last 16 years,there has never been a problem up until now that after this month it is going to stop as the 17 nearly 18 year old child is no longer in education.The ex wife is now saying she is going to take us to court to get csa back dated from 16 years ago,claiming she should of been getting more money in their agreement… even though she agreed to it and did for 16 years ,has she got any sort of claim on us??