(1)Where—
(a)the immediate employer of a worker is himself in the employment of some other person; and
(b)the worker is employed on the premises of that other person,
that other person shall be deemed for the purposes of this Part to be the employer of the worker jointly with the immediate employer.
(2)Where the commission by any person of an offence under section 16(2) or 19(4) is due to the act or default of some other person, that other person shall be guilty of the offence; and a person may be charged with and convicted of the offence by virtue of this subsection whether or not proceedings are taken against the first-mentioned person.
(3)In any proceedings for an offence under section 16(2) or 19(4) it shall be a defence for the person charged to prove that he exercised all due diligence and took all reasonable precautions to secure that the provisions of this Part, and of any relevant regulations or order made under it, were complied with by himself and by any person under his control.