This time it was a mill that wouldn’t run despite the rotor spinning freely, which suggested an electrical fault. After testing the obvious stuff such as the fuse and power cable it seemed as though there was no choice but to take it apart.
Once the motor was out of the mill body the problem became obvious. The safety interlock that only allows the motor to run when the chute is in place had broken.
Fortunately, spares are available. They’re supposed to look like the one on the right.
Fortunately replacement is just a question of popping open the switch body and screwing the cable into two terminals, so once the spare had turned up it was a relatively quick fix.