I was using studio flash - a couple of old Bowens mono-silver heads (no idea of their nominal output, but far greater than on camera flash). (The flash meter reading was f64 (unfiltered); lens aperture f4, and there was no image at all - histogram just a line at the black end)
I don't think the camera allows multiple exposures, so I can't try that.
I will try some more modern camera accessory flashguns, but need to get some batteries first, as I hardly ever use them these days, and the batteries I have are dead.
When I said the Finepix records IR through an SFX filter, I should have said through a number 87 filter. I'll check how many stops it loses next time I have some sunlight to play with!