According to 01.15 of the SRA Code of Conduct, solicitors must account to their clients for any commission received, regardless of the amount. Therefore, even a commission of £19.99 must be disclosed and accounted for. Options A and E are incorrect because there is no de minimis threshold for accounting for commissions. Option B is incorrect as the requirement to account for commission applies regardless of the specific arrangement. Option C is also incorrect because the rule applies to all types of commissions from investment-related business. 

Key Point: This question evaluates understanding of the SRA Code of Conduct's requirements regarding commission disclosure and accountability in the context of financial referrals and investment-related business.