4While, by virtue of an order under paragraph 3 above, paragraph 2 is excluded from applying to any work or to any specified part of any work—
(a)in any proceedings brought under section 1, 2 or 20 in respect of the emission of smoke from the work or (as the case may be) from the specified part of the work it shall be a defence to prove that the best practicable means had been employed to prevent or minimise the alleged emission;
(b)in any proceedings brought by virtue of section 17 (smoke nuisances in Scotland) in respect of smoke emitted from the work or (as the case may be) from the specified part of the work, the defence provided for by subsection (2) of that section shall be available whether the smoke was emitted from a chimney or not.