The arrangements must provide for the setting up, in relation to the underwriter, of a special reserve fund vested in trustees who have control over it and power to invest the capital thereof and to vary the investments :

Provided that where part of the business of the underwriter is carried on through an underwriting agent and part thereof is not so carried on, or where different parts of his business are carried on through different underwriting agents, the arrangements may provide for separate special reserve funds being constituted in relation to the different parts of his business.