I pulled out the heater fan and replaced it this morning.
Went to start the car, and there's a solenoid clunk from under the bonnet, followed by airbag AND ABS warnings from the trip computer.
What have I missed... broken... f*&^ed up?
Erm. Never mind.
Correlation != causation.
Flat battery will cause those faults. And stops the car from starting for some reason.