Court Oder variation / alteration

When I divorced my ex wife we agreed a financial settlement , the agreement was varied to remove maintenance payments for a capital sum. Under the existing agreement I’m owed equity in the former marital home. This equity triggered when our daughter turned 18 or stopped full time education . My daughter is now grown up and working, however the varied agreement now dosnt allow for my equity and to make matters worse my former wife is renting the house out as she lives with a boyfriend.
I’m looking to challenge the court order as this seems grossly unfair