Graham, it sounds as if the control itself has a problem on those two lowest settings, but there may be a broken or loose wire somewhere that is not feeding these low settings (I suspect this would be a 110-volt supply).   If you haven't already, take the back panel off the range and check if there is a loose or broken wire somewhere.  

Dang, I wish I had some service information on Moffatt stoves... 

Glad to hear the burners are ok  ;-)

I also can't help but think that there could be something with the power going from the switch to the burner... 

It's not easy to explain, but the way these old three-wire switches and burners worked was to tap across the 3 wires to provide 220 or 110 volts to a combination of inner, outer, and both coils (this has been my experience with the older GE and Frigidaire ranges).   The 'neutral' wire, may not always being used as a neutral.  As an example, if you take the cord that supplies the range, there will be a red, black, and white wire.  If you connect a meter to the red and the white OR the black and the white, you'll get 110 volts.  However if you connect a meter across the red and the black wires, you'll get 220 volts.  So, the 'neutral' is now either the red or black wire...  Based on what I've seen from other ranges, the lower heat settings are usually using 110 volts so this is why I thought perhaps there was a supply problem with the wires that are supplying the 110.  

If I remember, I'll try to post a photo of an example of a 5-heat switch from a range of this era on the weekend.  

Type of burner. When everything is working, you have a really flexible range of temperatures from lowest simmer to a really fast boil.

When something fails, it's very, shall we say, interesting to fix. That only one unit isn't working is helpful, though.

First, try running the problematic element on another switch. If it works fine there, then you can discard that possibility. 

Second, measure (carefully! high current!, hard to find controls!) the voltage condition to each of the wires going to one of the similar burners at each of the five heat settings. 

Third, when you compare it to your failed switch, you'll probably find the answer - it's almost certain that one of the internal contacts has burned through -but it could be a loose wire.

Now, if you can't get the replacement switch, not to worry. Worst case, a new burner and universal energy control will fix everything. Second best - you replace the switch with a new universal one (you'll have to match voltages and watts of the two coils, but doable).