Thanks Herco for your kind advice.
I have now compared the two lenses (10-24 mm vs 18-55 mm) at 24mm (because this is the narrowest field of view on the 10-24). This confirmed that there was nothing happening at all on the 10-24mm lens. It is strange because the camera body registers the OIS and there is the usual 'whirring' sound if one places the lens to the ear whilst the OIS is operational. Also, when the lens is not active (camera switched off or lens separated from the body), there is a slight rattle if one shakes the lens from side-to-side - this rattle disappears when the lens becomes active.
Your advice really helped - so now it's off to the local Fujifilm Service Centre (Australia) to sample their legendary customer service. I will post an update with the results.