If you are using a 12v coil, I wouldn't expect you would use a ballast resistor as well. IIRC, the Minx I had didn't have a ballast resistor and used a 12v coil as standard fitting. There are two reasons for using the resistor - 1: to use a lower voltage coil (as I had in my Maxi when I had electronic ignition with a coil from the same firm - Mobelec - to "suit" the electronic system) or 2: to use where there is a separate feed from the "start" position on the ignition switch that feeds to the coil side of the resistor and, when in "run" the coil feed is via the ballast resistor to the coil. This is so that there is a bigger spark when starting the car - and the coil doesn't burn out while it is running! The feed from the ignition would normally go to one side of the ballast resistor and then from the resistor to the coil so that the coil drops voltage.
I was assuming that you have positive earth (and the coil negative terminal goes to the ignition switch, the positive to the "points"). However, I had either not realised or forgotten that it has electronic ignition (ie no points). If it has electronic ignition, is it negative earth too? If so, the positive terminal on the coil needs to be connected to the ignition switch.
I wonder if the electronic bit has died and isn't "opening". Testing it may be easy (I haven't had a lot of experience with modern units - having had diesels for 24 years!). One way of testing the rest of the system would be to put a plug lead into the coil's HT terminal and a plug in the lead (earthed to chassis) and, with a flying lead from the "points" terminal of the coil, flash the coil to earth and watch for a spark from the plug.
I'd expect 12V and 10v at the coil's switch terminal - working the starter motor will make the battery voltage drop by a volt of two.
John's probably right about the capacitor (of course he is right!) - I was thinking "electrolytic" when I saw a round capacitor with a lead from the end
