My buyer pulls out of the contract and refuses to complete due to a material deterioration in the condition of my property after exchange of contracts?

You will not be able to trigger ClozeSure.

If the deterioration is due to your failure to fulfil your obligations under the sale agreement, you are effectively in breach of contract. This means you would almost certainly be liable to return the deposit to the buyer (who may bring a claim against you for the costs, damages and losses they have incurred due to your breach of contract). ClozeSure cannot be triggered by a seller who is in breach of their sale contract.

Note that you will not be entitled to a refund of the ClozeSure fee.