What to do
Leave the engine off, push the car clear of the pump in neutral, tell the cashier, and call for a mobile drain.
What happens if you start it
Diesel won't ignite from a spark plug. The engine misfires, smokes and usually stalls. Plugs can foul and unburnt fuel can damage the catalytic converter.
Can I top up with petrol?
No. Diluting doesn't stop diesel reaching the engine. The tank needs to be drained.