Out of interest the marker receiver was the 'fall-back' for complete comms receiver failure in my days in the RAF - and it worked. Airfield towers had an 'emergency' voice TX function available on their console.
BOAC, I think you'll find that the RAF 'Emergency' voice TX function used the ILS Localiser Transmitter. An aircraft with lost comm could hear the controller, rather than the ILS Ident, through a Nav Receiver.