In the debentures or stock of any local authority within the meaning of the Local Authorities Loans Act 1956 that are authorised investments for the investment of trust funds under the provisions of section four of the Trustee Act 1956:

“Provided that the Board shall not be entitled under this paragraph so to invest any money forming part of any particular fund of the Board, if at the time of the investment the total amount of the securities of such local authorities held pursuant to this paragraph or to paragraph (d) of this subsection as investments of that fund, together with the money proposed to be then invested, exceeds half the total investments for the time being of the money in that fund; or”.