Where a person designs, manufactures, imports, sells or supplies an article or substance for or to another person
and that other person undertakes in writing to take specified steps sufficient to ensure, as far as is reasonably practicable, that the article or substance will comply with all prescribed requirements and will be safe and without risks to health when properly used, the undertaking shall have the effect of relieving the first-mentioned person from the duty imposed upon him by this section to such an extent as may be reasonable having regard to the terms of the undertaking.