Hi Ray - to add to what AK has said it is worth remembering that the key requirements of HSWA only codified into criminal law what was already in effect in common law.
Then if we look at the full title of the Act it is the Health and Safety at Work etc Act 1974, such that unless you get into the "etc" bit there needs to be "work" as defined in HSWA Section 52, essentially meaning that somebody gets paid whether as an employee or in self-employment.
So, when you read Section 4 it applies to those in control of premises in relation to those who—
(a) are not their employees ; but
(b) use non-domestic premises made available to them as a place of work or as a place where they may use plant or substances provided for their use there
So, the underlined words appear to extend beyond "work".
...but this needs to be read in conjunction with Section 4(4)
(4) Any reference in this section to a person having control of any premises or matter is a reference to a person having control of the premises or matter in connection with thecarrying on by him of a trade, business or other undertaking (whether for profit or not).
But that in turn has to be read in conjunction with Section 1(1) of HSWA [which nobody often reads!)
(1) The provisions of this Part shall have effect with a view to—
(a) securing the health, safety and welfare of persons at work;
(b) protecting persons other than persons at work against risks to health or safetyarising out of or in connection with the activities of persons at work;
(c) controlling the keeping and use of explosive or highly flammable or otherwise dangerous substances, and generally preventing the unlawful acquisition,possession and use of such substances ; and
(d) controlling the emission into the atmosphere of noxious or offensivesubstances from premises of any class prescribed for the purposes of thisparagraph.
So, I suggest that Section 1(1)(c) and (d) in effect set out how the Act applies in terms of the "etc".
So, we had (c) which deals with potential big bangs and (d) which related to matters covered by HSE's long since lost Alkali Inspectorate - with all their subjects of interest now enforced by other enforcing authorities under legislation other than HSWA or its "relevant statutory provisions" (another defined term - but essentially health and safety legislation in force before HSWA and regulations made under HSWA).