The owners of any premises, if they are persons who under the [1965 c. 56.] Compulsory Purchase Act 1965 are empowered to sell and convey or release lands, may charge those premises with—
(a)such sum as may be necessary to defray the whole or a part of any expenses which the owners of, or any other person in respect of, those premises for the time being are liable to defray under the private street works code, and
(b)the expenses of making such a charge;
and, for securing the repayment of that sum with interest, may mortgage the premises to any person advancing that sum so, however, that the principal due on any such mortgage shall be repaid by equal yearly or half-yearly payments within 20 years.