The GSIU Regs do not apply to a factory, farm or construction site but inspections must be done under PUWER and HSW. However, the sprit of the regs and ACOP would apply - it being reasonably practicable to follow requirements for flues, fittings, installations etc.

You wouldn't necessarily need a Gas Safe Registered Engineer to carry out the work but you would need a competent person. In my experience it is easier to use a Gas Safe Registered Engineer with the required competencies, which can be checked on the GSR website, as it's easier.

Thanks for the feedback guys.
The landlord had these units serviced last November before we moved in to these premises and whilst the lease puts future servicing responsibility on ourselves, it doesn't state timescales.
We are a relatively clean workshop, with limited potential for airborne contamination of the units.
Also, they are heaters only, not air movers.
Based on the above, and the fact they are only used in cold periods, I'll include them on to our 2 yearly service schedule.