My role is now over, but I can tell you what needs to happen. You should point out to the other side (preferably through a solicitor’s letter) that they have breached the agreement. Their breach may be accidental and they may be unaware of it. This takes care of the problem cheaply, especially if it has not caused you immediate loss.
Remind me whether any court proceedings were running before the mediation. If not, and they do not respond to your letter as wished, you should threaten to sue for breach of contract (and then do so, if needed). The threat should be enough to bring them back into line.
If there were proceedings running prior to mediation, your agreement should have contained a term recording your mutual agreement to obtain a Consent Order from the court to bring the proceedings to an end on the basis of your agreed terms. I assume you now have that Consent Order. If so, you can ask the court to enforce it and the terms of your settlement agreement contained there (far easier than starting a claim). If the proceedings were stayed through a Tomlin Order, you can apply to lift the stay and enforce your agreed terms in a similar way.