tony why is that if the diode pack or the regulater within the altenator has failed short circit it is possible for the altenator to back feed the warning lamp circit therfore keeping the warning lamp on with ignition off there for engine stays running with ignition off.
have had this with a vauxhall about aa year ago.
garage changed the pump ignition switch and heater control before i looked at it and found altenator faulty strange thing it started and stopped with altenator warning cicit disconected and new alt cured fault so im sorry but ohh yes it can
i thought it bight be worth checking and no one else has posted any ideas ect and after all it doesent take long to check!

sorry nigel