Fuji wrote back to me to say that I should test the on/off switch against other cameras in the store where I bought it, with the offer to fix it if the resistance turns out to be different. Somehow I think the other cameras will exhibit the same characteristic.
Nevertheless, I have found an accessory grip by RRS (Really Right Stuff) that should resolve the issue, as it will provide a further surface to shield the switch, once installed.
Today, I went out briefly at lunch. I'm still familiarising myself with the camera but I came home with one acceptable result.
