http://www.legislation.gov.uk/uksi/1958/498/regulation/5/made
The Dark Smoke (Permitted Periods) Regulations 1958
Westlaw
King's Printer of Acts of Parliament
2011-04-05
SMOKE
Section 1 of the Clean Air Act, 1956, makes it an offence to emit dark smoke (as defined in the Act) from the chimney of a building, or of certain boilers or industrial plant, but provides that emissions of smoke lasting for not longer than such periods as may be specified by the Minister of Housing and Local Government by regulations shall, in such classes of case and subject to such limitations as may be so specified, be left out of account for the purposes of the section. These Regulations specify the permitted emissions of smoke, but do not apply to emissions of smoke from vessels.
5
Nothing in these regulations shall apply to the emission of smoke from any vessel.