5. Where the person concerned is or is to be treated as the sole owner of, or a partner in, any business, the value of such business to him or his share shall be taken to be either–
(a)such sum, or his share of such sum, as the case may be, as could be withdrawn from the assets of such business without substantially impairing the profits of such business or its normal development; or
(b)such sum as that person could borrow on the security of his interest in such business without injuring the commercial credit of that business;
whichever is the greater.