Firstly, assuming you still have the original Miller distributor, turn the engine until the points are open, remove the LT wire from the distributor, and put a low wattage bulb in circuit and check whether it lights when the ignition is turned on. If it does, there is a short circuit in the LT side of the distributor. The most likely culprit is that the spring blade on the points is shorting to the distributor body. Inside the body, there should be a thin fibre strip separating the spring blade from the body. Also, the hole for the terminal bolt should have 3 small fibre washers in it preventing the bolt from touching the body (these are very fragile and are often in a state of collapse). Finally there should be another fibre washer on the outside of the body, a plain washer and nut, then another washer and nut to secure the LT connection.
If all is OK with the LT side, check the HT side (the distributor cap). Undo the 2 screws that retain the clamp that secures the HT leads. Check very carefully for any signs of the spark tracking across the top of the cap (lead to screw to lead, in any direction). Any sign of tracking will cause the spark to travel to the wrong cylinder - the line of least resistance is always to the cylinder that is NOT under compression. If there is any signs of tracking, scrape away all the charred Bakelite and fill the groove with 24 hour Araldite to prevent further tracking. Also, check that the engine is timed on the correct cylinder (with the valves rocking - going from exhaust to inlet - on the LH cylinder - the RH cylinder should be firing) - quite a few Dragonflies are timed so that the HT leads cross (under the cover).
If you suspect the condenser is faulty, replace it with one for something like a Morris Minor - ignition condensers are not particularly voltage sensitive, so one intended for 12 volt will work on a 6 volt system. My bike has been running on a Morris Minor condenser for the last 20 years - I have, however, made a small alteration in that my condenser is mounted alongside the ignition coil and connected to the other end of the LT lead (I know they should be mounted as close as possible to the points but they definately don't like being cooked under that cast alloy distributor cover!).
Good luck,
Regards,
Eddie.