(1)In any proceedings for an offence under section one of this Act for any contravention of that section occurring not more than seven years from the passing of this Act, being a contravention occurring in relation to the chimney of a building, it shall be a defence to prove—
(a)that the contravention was due to the nature of the building or its equipment and was not due to any failure properly to maintain the building or properly to maintain and use the equipment of the building; and
(b)that it had not been practicable to alter or equip the building so as to enable it to be used or fully used for the purpose for which it was intended without the likelihood of contraventions of the said section one.
(2)If, at any time before the expiration of the said seven years, the local authority are satisfied, on the application of any person interested in any building, that it has not been practicable to alter or equip the building so as to enable it to be used or fully used as aforesaid, the local authority may, if they think fit, issue from time to time a certificate to that effect, and while such a certificate is in force, it shall be conclusive evidence of the facts therein stated for the purposes of paragraph (b) of subsection (1) of this section.
(3)A certificate under subsection (2) of this section shall be in force for one year from the date of the issue thereof or for such shorter period as may be specified therein, so, however that a certificate issued after the expiration of six, but before the expiration of seven, years from the passing of this Act shall not remain in force after the expiration of the said seven years.
(4)A certificate under subsection (2) of this section may, if the local authority think fit, be limited to particular chimneys of the .building and in that event the operation of the said subsection (2) shall be limited to contraventions in respect of those chimneys.
(5)This section shall apply in relation to a chimney serving the furnace of any boiler or industrial plant as it applies in relation to a chimney of a building:
Provided that, in relation to any such chimney as aforesaid which is not a chimney of a building, for the references in this section to the building and to maintaining it there shall be substituted references respectively to the furnace and any apparatus used in connection therewith and to maintaining or using the furnace and any such apparatus and the references in this section to the equipment of the building or equipping the building shall be omitted.