So this is how I imagine the ignition loom is set up...
The car has run perfectly fine for 4 or so months in this configuration - i.e. with an external Ballast Resistor prior to the ignition coil, and a single wire to the positive terminal of the ignition coil.
The coil itself is a 6V ignition coil, and specifies that a resistor must be used with it.
Putting a multimeter onto the Ballast Resistor terminals and the ignition coil terminals, I get the following voltages:
Ballast Resistor Postive : 10V when cranking, and 8V on run
Ballast Resistor Negative: 10V when cranking, and 3.9V on run
Ignition Coil: 10V when cranking, and 3.8V on run
Surely the ballast resistor positive should be receiving a constant 12V feed at all times. And 3.8V on the ignition coil seems way too low.
I am getting spark off the HT Lead from the coil to the distributor. Tried to bypass the ignition switch last night by removing the the ignition feed onto the coil, and running a cable directly from the battery positive but could not get the car to start.