Just to update- manage to sort this today, following everyones advice. All of the old 2 core and earth wiring removed, and I ran the 3 core and earth from the ceiling light (it was indeed already running from the light fitting), across to the isolator switch, and then onwards to the fan. Everything working perfectly and only had to cut two relatively small access holes in my bathroom ceiling!
No more extractor fans which work on humidistats or PIRs, for me. That's two now that I've had which after a couple of months stop working properly and appear to be due to moisture getting to the sensor itself....
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