Right I've had a look and can confirm there's no power to the coil while cranking.
However, playing with it a bit more, I think it's a warn / damaged starter switch.
If I turn the switch all the way to the stop then the engine cranks but will not fire (until you release the key)
BUT if I turn the key until it cranks, then back just a little, it will fire up while cranking.
So I guess it needs a new switch, which may mean it needs a whole new unit.
So for now it will remain a "to do sometime" job as I can live with it, particularly now I've found turning the key back a little will let it fire while cranking.