If you are using a solicitor for the sale of your Lot then check with them. You don’t want any future contract to ‘fall over’ due to an oversight in completing the sale contract details.
You may need to disclose the existence of the mediation issue as it is a matter that affects the property and could result in significant cost and inconvenience to the purchaser. This type of issue is not easily discoverable during the purchaser’s due diligence process.
The purchaser may be able to rescind the contract if the mediation was not disclosed by you, the seller. The purchaser could argue that they would not have entered into the contract had they had prior knowledge of the mediation.