hi nigel
just had a look on ad at the diagram there are 2 relays within the engine bay fuse box one is ignition main circit the othe is engine control relay would be wort testin them to see if they ar holding in or if they have gone short across pin 30 to pin 87
are the ignition/dash warning lamps switching off with the key and i preume you are stalling it to stop it when engine off is the ecu still recieving an ignition supply?
the only other thing i could see from the diagram worth checking is the red and white wire from the preheater control see if that is remaing live with engine off poss short internal of pre heater control unit
failing that you will need to check the wiring to the ecu to find what circit is not shutting down if you can pinpoint where to check next
hth martin