Ok, I went out and took the tank off for another go.
My switch

Grounding point.

The blue/yellow wire on the multi plug I was talking about.

Jumping the positive supply to the positive feed to the fan.

The result - no fan! I reckon my diagnosis of a dead fan motor is spot on!
I'm in negotiations for an alternative which should be here in a couple of days if all goes well!
I guess I'll be replacing it when the rains arrive this weekend!