It's a tricky one, Emmie, and will require professional legal advice that I can't give you. You also don't say what else the settlement contained. Are you entitled to profits from his business? What was he supposed to contribute to the house? What amount of child support was supposed to be paid?
But regarding the single issue you have asked about? On it's own, the court order is probably enforceable and you will either have to pay your ex one third of it's value. However my view is that this third will be a valuation of what it was when you separated, NOT what it is now.
So, if at the date of separation, (let's say February 26, 1988), the house was valued at �99k, then you need to find �33k. If the house is now valued at �300k my view is that he is not entitled to �100k - only the value at the relevant date of separation - �33k. That figure, I am sure, could be offset against whatever parts of the agreement your ex failed to meet. Perhaps you could even gain?
See a solicitor who specialises in Family Law but perhaps only after your ex decides to take action against you. Unless you feel you could gain from court action, then wait for him to try and enforce the sale/payment he is apparently owed.