Post Divorce Renegotiation?

My partner married and divorced outside of the UK. My partner is British, has two children (minors) and they live with us 50% of the time. The ex lives in the UK too.

A Clean Break settlement was done several years ago (my partner paid a cash settlement). However there are a number of new issues:-
1) My partner has lost all income and capital and is in financial hardship.
2) My partners ex now has significant income and capital, plus they did not disclose their income at the time of the settlement.
3) Their contract of settlement was drawn up outside the UK.

Can my partner renegotiate the original settlement so the ex can contribute to the children's needs when they reside with us (50% of the time)? And if so what would be the best route? CMS? Solicitor? Mediator? It is an accrimonuous relationship and an initial approach to contribute has been ignored.

A clean break ends all future claims. That’s the idea of it. What information was withheld at the time? Was it likely to make a difference at the time.

As far as child maintenance is concerned you could apply to CMS but you’ve mentioned that it is 50/50 shared care. Who claims child benefit?

Thank you - the clean break was completed overseas as an agreement rather than court ordered.
Nobody claims child benefit as far as I am aware