Thanks mars23 - that worked a charm

I managed to reset the low range flashing by putting my ML350 (2004) into neutral and hitting the low range switch twice - once to engage it and then again to disengage it. I could feel and hear it cycling through. I then restarted the car a couple of times to test that the dash light didn't come back on and so far so good...and just in time for the snow.

Note to future troubleshooters - the car must be in neutral, not park to re-cycle the switch.