As already suggested, insert a bulb in the LT circuit and check that it goes on and off as you open and close the points. If the light stays on contiuously, there is probably a fault in the insulation where the return spring is bolted through the distributor body. If that is OK, remove the bulb and check for a spark at the HT lead from the coil. Again, if that is OK, carefully check out the distributor cap - the actual cap has a top plate that clamps down the 3 HT leads (the centre one from the coil, and the outer 2 to the spark plugs) - between these leads there 2 2BA bolts that retain the top plate. If moisture has been allowed to get into the joint between the cap and top plate, the spark may have shorted between the centre lead and one of the others - via one of the bolts (sparks always take the line of least resistance, and that is to the spark plug that, at the time, is NOT under the compression of the engine). Close scrutiny with a magnifying glass will probably reveal a fine black line - a burn between 2 of the leads - carefully scrape at the line until it is no longer visible, and put a good smear of Vaseline in the joint to prevent further shorting. You should now have a spark at the plugs - if not, your condenser is probably suspect - in use, they can get overheated under the alloy distributor cover - I mount mine on the other end of the lead to the coil, so that it gets a good blast of cooling air. A good replacement condenser is one from a Morris Minor - they don't seem to be voltage sensitive.
Regards,
Eddie.