If a mortgage, charge, pledge or other security is provided by a person to secure the obligations of a purchaser under a contract of supply of goods or services, the amount secured in respect of the liability of that person in respect of those obligations does not exceed liability in respect of the sum of—
(a) the amount for which the purchaser is liable by reason of the breach of the contract of supply of goods or services; and
(b) the reasonable costs of, and incidental to, enforcing the mortgage, charge, pledge, lien or other security.