D was smoking on a train where it had been banned under delegated legislation but argued that that ban exceeded the powers conferred on the PA that issued the by-law. HL held the ban as lawful. However it noted that in criminal charges it is a defence to claim that the delegated legislation criminalising the act in question is ultra vires (i.e. unlawful, whether this was on the basis of procedural or substantive review).
Lord Lairg: “only the clear language of a statute could take away the right of a defendant in criminal proceedings to challenge the lawfulness of a byelaw or administrative decision where his prosecution is premised on its validity.”