I have checked the fuses visually, and they seem in perfect nick, but
I'll continuity test them properly in a bit. Since I'm sitting with a
cup of tea in the warm at the moment free to consider theory, what else
should I be looking at sternly and fundamentally how can I test the
operation of a thermostat operated bit, rev it till steam comes off?

I can, plonk 12V on the motor and see if it spins. I can continuity test
the wiring any place I find a metal bit to pop a multimeter on.

Is there anything I can do with the thermocouple screwed into the rad.?
unscrewing it would get me wet feet I suspect.